Before Gorbachev, the previous leaders of the Soviet Union crushed dissent and any threat to their sphere of influence (East Berlin in 1953, Hungarian Revolution, Prague Spring, the 1979 invasion of Afghanistan, and indirectly the Derg in Ethiopia). When Gorbachev came in, he implemented glasnost and perestroika, then told the eastern bloc to listen to their people and not crush them. In less than a decade, the Cold War ended without a nuclear apocalypse or even much bloodshed (with a few exceptions). How did a reformer like Gorbachev come to power, and did the hardliners in the Communist Party try to stop it?
The CPSU always had different factions within it. Gorbachev was a career politician, he was just always a reformer. By the 1980s the core leadership of the CPSU were still essentially Stalin's old cronies, but after 40 or 50 years they were 70 and 80 years old. Brezhnev, the last major Soviet leader before Gorbachev, was like 80 years old during his administration. Gorbachev meanwhile was only in his 50s. The general aging of the old guard forced them to bring in new blood, many of whom were reformers.
And as for if the old guard tried to stop it, yes they did, and they failed. See here:
http://en.wikipedia.org/wiki/1991_Soviet_coup_d%27%C3%A9tat_attempt