WARNINGS of a coup have sounded throughout Mikhail Gorbachev's six years of reforms. Hard-liners whose orthodoxies were threatened by Mr. Gorbachev's policy of openness grumbled, but they held back, seeing no alternative to his leadership.That has now dramatically changed. In the best tradition of Kremlin evasion, Gorbachev was declared in ill health early Monday morning and Vice President Gennady Yanayev was handed the reins by a junta-like committee headed by the military and KGB. The implications are immense. President Bush's "new world order" hinged, to a large degree, on changes wrought by Gorbachev. Is the old order asserting itself? Will the pivotal event of the '90s be the reversal of the pivotal event of the '80s? The hard-liners acted as the new Union Treaty between the center and nine Soviet republics was about to be signed. The treaty would have shifted a large part of the responsibility for the country's economy toward the republic governments. That was more than the hard-liners could stomach. They claim the economic changes begun under Gorbachev will continue. But that pledge will have to be tested. The impact outside the Soviet Union is immediate and extensive. NATO, which had been shrinking in importance as its adversaries to the east fragmented, suddenly resumes center stage. Any concerted Western response to the coup is likely to come through the alliance structure. Eastern European countries, just getting used to autonomy, feel the old concerns returning. The Middle East peace process, backed by the Soviets and the United States, could be jolted. And what does this mean for the flow of Soviet Jews to Israel? Still, the reforms that came with glasnost won't be reversed by decree. People have grown used to speaking their minds and to having a say about their leaders. Boris Yeltsin, president of the Russian Republic and the country's most popular politician, has called for a national strike to protest the action. The international response to events in Moscow should be unequivocal. The reforms begun by Gorbachev point the only reasonable path toward Soviet economic revival. Their continuance has to be firmly endorsed. Any inclination toward violent repression must be firmly denounced. The hard-liners hope to use the public's economic stress to legitimize their actions. But the Soviet people may not be willing to trade their new freedoms for the same old communist promises.