The Prague Spring itself was a period of relative freedom during 1968, when Alexander Dubcek allowed more freedom in writing and living within Czechoslovakia. It went on for months, from January to August 20th 1968, when Soviet tanks invaded.
The Soviets invaded in response to Dubcek's "socialism with a human face" policy, which made it appear as though they were losing power. In order to reestablish it, they went in with their tanks. Following the Prague Spring, people were much too scared too act. This event is why citizens did not take any action for 20 years.
The years following are commonly referred to "The Era of Forgetting." People remembered it strongly however when the self-immolations of Jan Palach and Jan Zajic made an impact after 20 years in 1989.