What was going on when I was born?

Four Soviet diplomats riding in official cars were kidnapped in West Beirut today, authorities said. It was the first abduction of Soviet nationals in the Lebanese capital. As is usually the case in such kidnappings, no one took responsibility. There was no immediate official response from the Soviet Union.

Mayor Koch yesterday ordered an investigation of what he called possible ''incompetence or corruption'' in the construction of the most expensive public high school ever built in New York City. He acted after new disclosures of ''structural cracks'' and other design defects at the school. The Mayor also made public a letter he sent to Schools Chancellor Nathan Quinones on Sept. 26 asking that Mr. Quinones and other Board of Education officials provide ''an account of what steps you are taking to recover damages for faulty design and faulty construction'' at the school, the Fiorello H. La Guardia High School of Music and the Arts. $75.8 Million Spent The school, built at a cost of $75.8 million, opened in September 1984. But it remains unfinished and troubled by numerous design problems that have forced the board to curtail some classes for safety reasons and to order reconstruction and other work that is expected to add more than $9 million to the final bill.

An article yesterday about the Metropolitan Museum of Art's plans for a $35 million structure gave an incorrect date for the Landmarks Preservation Commission's consideration of the project. The plan will be considered in executive session ''sometime in the near future,'' according to Gene A. Norman, the commission's chairman.

The United States Public Health Service today unveiled a long-range plan to control the spread of acquired immune deficiency syndrome, or AIDS, that acknowledges that no vaccine or cure is likely for at least five more years and that the disease would therefore continue to spread until the turn of the century. The plan, published in the September-October issue of Public Health Reports, the official journal of the health service, sets three goals for controlling AIDS and the dates by which they are to be achieved. It is the first time the Federal Government has set target dates for countering the epidemic. The first goal, to be achieved by 1987, is a reduction in the steady increase in the transmission of the AIDS virus from individual to individual. An estimated 500,000 to 1 million Americans, mostly homosexuals, are believed to have been infected with the AIDS virus, and the total may be climbing by 1,000 to 2,000 per day, according to some unofficial estimates.

Edward E. Faber, the new president and chief executive of the Computerland Corporation, rushed today to forestall a revolt of the computer retailer's franchisees by promising a sweeping corporate reorganization and cuts in the fees the franchisees pay to corporate headquarters. Mr. Faber's actions followed the sudden resignations late last week of William H. Millard, the founder and chief executive of Computerland, and his 27-year-old daughter, Barbara Millard, who had been the president and chief operating officer of the family-owned company. Computerland is the world's largest computer retailer, with more than 800 stores worldwide. The Millards, who still own almost all of Computerland's stock, relinquished their operating responsibilities in a last-ditch effort to prevent the franchisees from filing suit or taking other action against the chain. The franchisees had been angry about Mr. Millard's autocratic management style and about the 9 percent of their sales they had to pay to corporate headquarters, an amount they have said was too high.

A listing on Sunday of food establishments cited for violations included an incorrect mention, provided by the City Health Department, of the Box Tree restaurant at 250 East 49th Street. The restaurant corrected four earlier violations; it remains open.

President Reagan plans to meet next month in New York with the leaders of six industrialized nations to discuss his summit meeting in November with the Soviet leader, Mikhail S. Gorbachev, the White House announced today. Those invited to the meeting are the leaders of Britain, France, West Germany, Canada, Italy and Japan, the White House said. A White House spokesman, Edward P. Djerejian, said the allied talks would focus on arms control as well as East-West economic and political questions. The meeting will take place Oct. 23-24, while Mr. Reagan is in New York to attend ceremonies commemorating the 40th anniversary of the United Nations.

The Soviet Union formally proposed today that the United States reduce its long-range and medium-range nuclear weapons by 50 percent, while offering a reduction that would cover only Soviet long-range weapons, according to American officials here. These officials said the Soviet proposal in Geneva made such cutbacks conditional on a ban on research into space-based defenses, which the Soviet Union calls ''space strike weapons,'' and a ban on testing or deployment of space-based defenses or anti-satellite weapons. Retreat From Earlier Hints The officials here said the proposal was regarded in Washington as a retreat from recent indications - in a Time magazine interview with Mikhail S. Gorbachev, the Soviet leader, and comments to a group of United States Senators visiting Moscow - that the Kremlin might accept basic laboratory research on antimissile defenses. Another central element in the Soviet proposal, officials here said, was a ban on deployment of new strategic weapon systems. American officials said this appeared to allow Moscow to proceed with deployment of two land-based intercontinental missiles, the SS-24 and the SS-25, but to bar the United States from deploying the land-based MX and Midgetman missiles, the submarine-based Trident D-5 missile and the Stealth bomber, which are at earlier stages of development.

OVER the last 10 years, Jesse L. Smith has become more educated and more in debt. Despite what he describes as generous grants, work-study programs and scholarships, he will owe $30,000 by the time he receives his doctorate from Rutgers University next year. The first payment will be due in six months. Because of this debt, Mr. Smith says, he has pretty much ruled out a teaching career or some of the kinds of social work that most interest him. ''I'm going to have to get a job that is fairly high-paying,'' he explained, ''and I'm going to have to include the payments in my monthly bills for the rest of my life.''

AS the costs of space vehicles, particle accelerators and telescopes soar out of sight, scientists are turning back to the humble balloon - little changed since its invention two centuries ago -as a cut-rate tool for exploring the origin and nature of the universe. At the National Scientific Balloon Facility (N.S.B.F.) in Palestine, Tex., balloons large enough to enclose entire football fields are lifting unmanned scientific payloads of two tons or more to the top of the earth's atmosphere. There, at altitudes of up to 30 miles, advanced instruments are helping scientists determine what the universe was like in the first few thousand years of its existence and how the matter of which we ourselves are made has evolved since then. At ''float,'' or cruising, altitude the 50 or more big balloons launched each year from the rolling green farmland of East Texas hang suspended in the golden boundary layer between the earth's atmosphere and space. Above them the sky is black and studded with stars even at midday, and below them is the sunlit, azure sea of air.

President Reagan today asked Margaret M. Heckler to give up her post as the Secretary of Health and Human Services and to become Ambassador to Ireland. Larry Speakes, the White House spokesman, said that Mrs. Heckler, in a private meeting with the President, asked Mr. Reagan for a few days to think about taking the ambassadorship, and that he agreed. Stops Short of Dismissal Mr. Reagan's action stopped short of a formal dismissal, and some White House officials portrayed it as a ''promotion.'' But for several months, in the face of pressure from some White House aides, Mrs. Heckler has been struggling to keep her Cabinet position and the responsibility for the Health and Human Services Department's $330 billion budget, the largest of any department.

The Government index devised to foretell broad moves in the economy rose a substantial seven-tenths of 1 percent in August, the fourth consecutive monthly gain, the Commerce Department reported today. The advance in the index of leading indicators was the latest in a series of reports that, taken together, have prompted many analysts to assert that the economy is gradually picking up momentum after showing an anemic growth rate of just over 1 percent during the first half of the year. ''I view it mainly as giving added confirmation that a modest acceleration of economic activity is indeed in progress,'' said Timothy Howard, chief economist for the Federal National Mortgage Association. Upward Revision for July The Commerce Department, which computes the index, also said that it had revised upward the July reading, which now also shows a gain of seven-tenths of 1 percent instead of the four-tenths advance previously reported.