Well, what happened to the great Ronald Reagan boom, fed by ''supply-side'' economics and undergirded by a deregulation revolution that was supposed to ensure unending prosperity?

It's not too much of a mystery: the huge national budget deficit and the pile-up of debt here and in the Third World have taken their toll. Economic growth in the United States has sunk to the 2 percent level, and business profits, despite the Reagan tax cuts, are shrinking.

Says a key administration figure in a private conversation: ''I don't think you can ever get to the substantial growth that we've been talking about until you get the deficit and the debt under control.'' Actually, the stagnation in the American economy has been going on since the end of 1984, covered up by glowing forecasts from the administration. Go back and read the self-congratulatory words issued at the May Tokyo summit.

Around the globe, despite the signs of stalling in the American economic engine, West Germany and Japan doggedly pursue a mercantilist, export-led prosperity. Thus, they are avoiding their obligation to take up some of the international slack by domestic expansion.

Meanwhile, the situation in some Third World countries borders on the desperate. Mexico -- in effect, an integral part of the U.S. economy -- cannot pay its annual service charges on a $100 billion debt at the present high rates. To maintain even a curtailed schedule of debt repayments, Mexico -- like other Latin American debtors -- has had to cut imports from the United States, adding to the woes of American exporters, including farmers.

And throughout the Third World, commodity prices have fallen as the slump in the industrial nations curtailed demand for products that are mined or grown in the poorer nations.

The drop in the stock market has raised the public's consciousness about the fragility of the economy -- domestic and global -- and given credibility to a boom-and-bust scenario. One contributory element to stock market uncertainty consist of question marks surrounding tax reform. But within a few weeks, the House-Senate conference will settle on a dramatic revision of the tax law, and businessmen once again will at least know how to plan.

Perhaps the most worrisome thing is that the sharp decline in interest rates in the past year has completely failed to stimulate a critical sector, business investment in new plant and equipment, even though lower rates helped the home-building industry.

Some experts are beginning to doubt that further cuts in interest rates will have much beneficial effect. For example, economist Alan Greenspan, an unofficial adviser to the Reagan administration, said in an interview:

''We may be getting to the point where business is reluctant to expand plant and equipment with borrowed money for fear that if the investments should not be productive, they couldn't pay the service costs on the money they borrowed.

''We may be at one of those periods in history where policy has very serious limitations. You hope that positive elements emerge. But my concern is that much of what policy can do is already in place.''

Others, like New York economist Henry Kaufman, still look for benefits from an easier money policy. The administration, with one eye cocked on the November elections, has also been pushing hard to get the Fed to lower rates: there have been three half-point reductions in the discount rate so far this year, including the one last week that dropped the rate to 6 percent.

In any event, almost everyone agrees that it will take more than an interest-rate fix to get the global economy going again. For one thing, the stalemate in international policy coordination must be broken.

And correcting the trade deficit obviously will take more than a dollar adjustment. It will require some changed attitudes by business and labor, and a new national attention to education programs that will contribute to productivity gains.

Meanwhile, Congress and the administration, in the wake of the Supreme Court decision negating the heart of the Gramm-Rudman-Hollings law, must act responsibly to correct the long-term negative impact of the budget deficit. Budget Director James Miller now estimates that the fiscal 1986 deficit will be a record $220 billion, because revenues were overestimated and defense spending underestimated.

To put the best light on all of these issues, we are in a mixed, murky situation, with listlessness the dominant tone. ''We are not about to go over the cliff,'' a high Reagan administration official insists. But the economy rarely stays in an indecisive state very long. It either will recover, under the belated impetus of lower interest rates, a huge Keynesian budget deficit, lower oil prices and a weak dollar -- or it will stall out.

For the best chance of averting that unhappy result, political leaders here, in Japan and West Germany have to get their act together, remembering how interdependent the big powers are with the poorer nations.