Suddenly, or so it seems, the world is being told that West Germany's miracle economy has no more wonders to offer.

A German slowdown, already discernible earlier in the year, has worsened and now threatens to bring growth in the national economy -- long-regarded as Europe's powerhouse -- to a standstill next year.

The problems this time go deeper than an easily dismissible cyclical downturn. Today's difficulties are symptomatic in substantial part of the middle-aging of the West German economy.

The loss of economic vitality has squelched, at least for the moment, Chancellor Helmut Shcmidt's pretensions of an expanded world political role for Bonn and, in time, could force the unlacing of the thick social net which the government has knit for its burghers.

Some of the main economic indicators that in the past have testified to Germany's model performance are still the envy of Americans -- 5.5 percent inflation this year, 3.8 percent unemployment, the world's richest currency reserves -- and point to a reservoir of reamining strength.

What is troubling, though, are a host of other alarming indicators:

Germany's current-accounts deficit worsened this year, sliding down to a projected year-end figure of nearly $16 billion. That is about three times last year's deficit, which was the first since 1965. The red-ink numbers are due mostly to the high cost of imported oil, which fuels half of West Germany's energy needs, and to the heavy-spending habits of globe-trotting German tourists. To help finance the deficit, Bonn has had to borrow this year from the United States and wealthy Arab states.

Also chiefly as a result of the deficit, the formerly invincible West German mark has had a vincible time on foreign exchange markets, actually slipping this year against all European currencies except the lira and falling about 7 percent against the dollar.

The threat of foreign competition, especially from the Japanese, is rising in West Germany's own home market. Most alarming has been the surge this year in Japanese auto markets, which have more than doubled their market share up to 10 percent. [On Wednesday, NISSAN AND volkswagen reached a broad agreement to manufacture VW passenger cars in Japan.] The Japanese already have claimed major shares of the electronic and photographic equipment fields here.

The once finely tuned relationship between labor and management in Germany came in for a battering this year in fights over worker codetermination in steel companies and the use of lockouts against striking workers. Though differences have been patched up for now, commentators have proclaimed the beginning of the end for Germany's long era of relative labor peace.

The economic ministry recently lowered to 2 percent its projection of growth for 1980, and leading forecasters said Germany could expect a growth rate next year of half a percent at best. Moreover, unemployment is projected to pass the one million mark in 1981, climbing from its current 880,000.

Other countries facing similar slowdowns would consider pumping up federal expenditures or lowering interest rates to give the national economy a spur. West Germany authorities feel themselves constrained from doing either.

Bonn's fiscal expansion of two years ago, done at the nudging of the United States and West Germany's other major industrialized partners, ran up the national debt, leaving Schmidt and his center-left coalition government with little room now for antirecession action. The conservative opposition party built the size of the federal debt -- the equivalent of more than $115 billion -- into its most effective campaign issue during recent national elections, making the public especially sensitive to federal spending policy.

Meantime, the Bundesbank, West Germany's central bank, appears intent on keeping interest rates high to attract foreign capital and help offset the current account deficit.

A return to economic vigor will depend both on things West Germans can themselves control ( moderation in wage demands this winter, cutbacks in oil use, restraint in government spending) and on things they cannot (the price of OPEC oil, and economic recovery in the United States and other cvountries with which West Germany does business.)

In the short term, Germany's energy and labor problems and its current-account and budget deficits appear managable. Export trade should remain the key to the country's industrial vitality so for some time. remain the key to the country's industrial vitality so for some time.

Despite heightened competition abroad, German goods and services are still valued for their high quality, timeliness of delivery and after-sales servicing. Germany's particular strength in the electrical and mechanical engineering fields and in the service sector leave it well-positioned in some expanding overseas markets, most notably in the Middle East.

But lagging growth and productivity suggest that for the German economy to perform well in the long term, fundamental shifts in government spending and business investment will be necessary to spark innovation, ease job adjustment for retrained workers, cope with an aging and declining population, and meet the mounting bill of social benefits. These policy changes are not yet in evidence, or even under much public discussion.

Attitudes also will have to change. There has been grumbling for some time that German workers are not what they used to be -- that the trains no longer run on time (they don't always) and that German workers have become overly pampered.

Regardless of the warning signs, major departures in policy and attitude are considered unlikely without a crisis. Taking a determinedly positive view of the future, the Bonn government's council of five economic advisers (known as the Five Wise Men) in mid-November labeled the country's current slump not as a recession but as merely a "growth pause."