Patrick Swayze. Huge as the 1980s became the 1990s, but after Ghost and Point Break I guess he decided to spend more time living his life than appearing in films.
Juliette Lewis? The 1990s? Juliette Lewis seemed to be all over the place in the 1990s, but she evaporated into thin air thereafter. Not necessarily a bad thing. Not strictly speaking on-topic, though, because you can tell why the roles dried up; she become famous playing teenage ne’er do wells, which she couldn’t continue doing as she got older.
Most of the cast of Young Guns (1988). Lou Diamond Phillips and Emilio Estevez were hot young stars at the time. Phillips’ career immediately tanked - it looks as if he was only given Latin / Native American roles - and Estevez seemed to stagger on for a while before giving up in 2000, probably decided to retire on the money he made the Mighty Ducks.
Moving back, Steve Guttenberg. Major a-list star of Police Academy, Cocoon, Short Circuit, 3 Men and a Baby. Bigger than Tom Hanks. And then nothing of note, in fact he became a kind of anti-star, a bad joke. In fact Tom Hanks is a kind of inverse of this topic, he prevailed where Guttenberg and Reinhold and Feldman and Galligan and Broderick failed. He beat them all.
Heath Ledger. Massive hit as the Joker in The Dark Knight, critical respect with Brokeback Mountain, but apart from a short part in some Terry Gilliam movie, nothing. And yet the producers of The Dark Knight didn’t make the same mistake Tim Burton made when he killed off Jack Nicholson’s Joker in his version of Batman.
Mind you, you can see why Scarlett Johansson became a star; she looks like one. She attractions attention on the red carpet; newspapers put her on the cover; free publicity.