Reposting an earlier answer I wrote:

You might be interested in checking out this answer I wrote previously on the role Gorbachev played in the dissolution of the USSR.

In summary, Gorbachev's reforms are ultimately responsible for the Soviet collapse, and ironically much of the argument that the Soviet economy needed change to avoid collapse came directly from him - the phrase "Era of Stagnation" is actually a piece of Gorbachev's rhetoric.

However there seems to be a strong case (made by Stephen Kotkin in Armageddon Averted), that while the Soviet economy was growing at ever slower rates, and increasingly unable to close the ever-present gap in living standards between the USSR and the West, probably could have continued to muddle on - there was no imminent danger of political and economic collapse in 1985.

It's also important to note that Gorbachev's reforms did not cause the collapse of the USSR on purpose, and Gorbachev was always committed to maintaining the union in some reformed shape under an economic system that was still socialist. However, his reforms both began to pick apart the centralized economy without really creating new institutions, which caused severe economic disruptions, and his political reforms unleashed new political movements outside his control, while all of these reforms antagonized more hardline members of the nomenklatura (party establishment). Ultimately he lost control of the situation.

Addendum: the fall of the Eastern European communist regimes in 1989 is also directly the result of Gorbachev's action, or more accurately inaction. He refused to continue the "Brezhnev Dcotrine" of enforcing communist Orthodox adherence by threat of Soviet military force, even when leaders of the Warsaw Pact states would have preferred to have this guarantee. Again, it's not so much that Gorbachev expected all of these regimes to swiftly fall and be replaced with non-communist ones. If anything he was trying to encourage the existing ruling parties to reform themselves and their societies in the manner he was attempting for the USSR (which was in large parts aiming for a democratic socialism or a "socialism with a human face" as reformers during the 1968 Prague Spring had urged). Gorbachev was just as shocked as other world leaders (and make no mistake, the events were swift and unforseen), but was unwilling to intervene to stop it.