S06E08: Rip Van Flintstone

Bored by his company picnic, Fred slips away to take a nap, and awakens twenty years later to discover that Barney has become millionaire B.J. Rubble, and that Pebbles and Bamm-Bamm have gotten married. Upon entering Wilma who is now living alone through Barney's largesse, Fred really awakens and realizes it was all a dream.