It caused many Marxist parties to collapse themselves or cease to continue to function as revolutionary leftist parties, with many of them opting to dissolve into some type of broad organisation or to continue existing independently but without a revolutionary perspective (AKA reformism), as had already been going on in Europe for example with the euro communism movement.
These trends are called liquidationism and revisionism respectively, and both are generally seen by revolutionary Marxists as deeply damaging to the movement.
From the 1970's onwards there had been a significant drift away from viewing the USSR as the focal point for the construction of communism. Disparate theoretical and ideological strands that took issue with the USSR's self-appointed leadership of global socialism in this period became known as "Eurocommunism". Whilst some factions and parties (Such as the CPGB in the UK) remained staunchly pro-Soviet as did many rank and file party members, many of the most interesting and influential theorists had already been arguing for a different sort of communism, using theoretical frameworks that were profoundly influenced by the Italian dissident Antonio Gramsci (who developed the concept of "Hegemony" which supposed the capitalists kept power through ideological and cultural means as well as just economic power and control of the state). Giorgio Napolitano of the Italian communists was a prominent moderniser within the european communist movement, later in his career he was viewed as such an establishment figure he became President of Italy. So, many communist theorists in Italy, France and Spain ended up viewing the collapse of the USSR as unexpected, but inevitable in hindsight, they had been criticising and diagnosing its terminal disease for many years.
Gorbachev came to be seen as a positive actor by many of these European leftists, especially as his reforms became more radical, however the more traditionalist left was aghast, many in the CPGB for example felt like they'd been cut adrift, when Gorbachev and Shevardnadze began outlining their "new thinking" which severely limited the effect the global "class struggle" between the "rival camps" had on foreign policy, it spelled the end of Soviet financial and political support for the Western European communist parties. Tariq Ali's novel "Redemption" dramatises the schisms and confusion of the British hard left in this period, whilst it takes dramatic and satirical license, it gives a good flavour of the times because Ali was an influential figure in the British hard left and a prominent publisher. Many Leftists of that time ended up becoming neo-thatcherite social democrats, this guardian article on "Marxism Today" gives a sense of how the British left viewed those times http://www.theguardian.com/politics/2015/sep/29/marxism-today-forgotten-visionaries-whose-ideas-could-save-labour