Nah, they didn't lose the blueprints. They decided there was no reason to go back to the moon because there was no Soviet Threat and no one to race against. The 70's was a time of gas crunches, high inflation and taxes and low employment. The people just didn't want to pay for the moon bases. And when Congress cut off NASA funding for moon missions, there was no reason to keep the hundreds of thousands of people across the country that build the individual components for the S-5 boosters employed, so they all went on to bigger and better things, or retired altogether. In either case, what that meant was, a brain drain. No one was left that knew how to build the things. Once the Shuttle program began in the mid to late 70's, everyone had space station on the brain, no one wanted to go to the Moon anymore, that was "So Last Decade".
The Orion booster is a direct descendant of the S-5 boosters, with a lot of modern upgrades learned form the shuttle program and other sources. But the bones are still there.