What was going on when I was born?

By dawn it was apparent for miles around that the long-awaited day had finally arrived. Clouds of bluish-white smoke billowed from the carbon-caped stacks. And gleeful but skeptical workers, some returning from months of unemployment, asked, ''How did it go?'' as they ambled through the plant gates. For the first time in 21 months, one of the aging blast furnaces at the United States Steel Corporation's Edgar Thomson Works is smelting iron ore, coke and limestone into pig iron. Shortly after 4 o'clock this morning, dozens of workers on the graveyard shift gathered around the furnace to watch the lighting, hoping they were witnessing the start of a lasting recovery in the feeble steel industry.

IT did not take quite so long to assemble ''The Great East River Bridge'' exhibition that opens tomorrow at the Brooklyn Museum as it did to build the bridge that is its subject, but the show is a singular accomplishment nonetheless. It is clearly the definitive event to mark the centennial of the Brooklyn Bridge - short of the celebrations that will occur on and around the bridge itself May 24, the anniversary of its opening. But those festivities will go on for only a day or so, while ''The Great East River Bridge'' will be with us all the way through June 19. That is a good thing, because there is no reason why anyone with the slightest passing interest in Brooklyn, New York City, bridges, technology, urban development, engineering, 20th-century painting, 19th-century art or photography should miss this exhibition. And three months should be time enough to make sure that everyone with an interest in any of these areas finds the time to get to the museum on Eastern Parkway.

France lost about $3 billion of foreign-exchange reserves in the week ended last Thursday as it struggled to support the declining franc and avert another devaluation, according to figures issued today by the nation's central bank. The loss is one of the largest ever experienced in a single week by the Bank of France. Meanwhile, the franc remains under pressure, with the Socialist Government still debating whether to devalue it a third time since taking power in May 1981 and introduce austerity measures meant to curb inflation and reduce the country's huge trade deficit. This continuing disagreement over economic policy has already forced President Francois Mitterrand to postpone a Cabinet reorganization originally scheduled to be announced last Tuesday following the Socialists' weak performance in municipal elections earlier this month. Paris is swept by rumors about which ministers might stay in office and which might depart.

Trading in shares of the Baldwin-United Corporation remained halted on the New York Stock Exchange yesterday for the third consecutive session and two of its insurance subsidiaries were fined $100,000 in Arkansas for violation of state insurance law. Meanwhile, in a reflection of growing concern over the unfavorable publicity the company has been getting in recent weeks, Kidder, Peabody & Company told its salesmen yesterday to stop selling Baldwin's insurance products, including its annuities. The move leaves Merrill Lynch, Pierce, Fenner & Smith Inc., Baldwin's largest seller of such annuities, as one its few remaining important outlets. Baldwin's stock last traded at $23.50. The cash-short financial services company continued to negotiate a further extension of $440 million in loans originally due Baldwin's lenders last Monday. A 12-day extension until March 28 was granted Wednesday.

The International Business Machines Corporation, continuing its aggressive moves against both domestic and foreign competitors, filed suit today against an American rival and five of its own former employees, charging theft of trade secrets. I.B.M. accused the Cybernex Corporation of San Jose, Calif., and the five former I.B.M. executives who founded it of misappropriation of trade secrets and unfair competition. The complaint, which seeks an injunction and unspecified damages, involves the principal component of a computer disk storage device known as the ''head.'' The component is used to ''read'' and ''write'' data on the disk's surface for storage.

There is seldom anything poignant about an oilproducing facility. But the words scrawled on a giant ball valve at the central compressor plant situated above the vast Prudhoe Bay oilfield tell of a dream deferred: ''To future gas plant 1986-plus.'' The gas plant, intended as the final processing point for Alaskan natural gas before moving it south by pipeline, has never been built. Neither has the pipeline. The original goal - to have it in service this year - has slipped to 1989 at the earliest, and probably far beyond. The reasons include the persistent abundance of oil worldwide, a similar abundance of far cheaper gas in the United States, the steep rise in estimated costs and severe financing problems.