Old Timer is right that bluegrass became much more popular as a result of the folk boom of the early 60s, but I don't think of it as a revival of bluegrass. Bluegrass had been growing pretty steadily throughout the 50s among a segment of country music fans who liked the down home feel and the acoustic sound, but not many northern musicians were interested in it until the 60s when bluegrass bands began appearing at folk festivals and folk singers showing up at places like the Old-Time Fiddlers' Conventions in Union Grove, N.C. and Galax, Va. Both of these had been going on since the 1930s, and you could hear plenty of bluegrass as well as old time clawhammer banjo picking at both of them.