My two candidates:
The Dixie Chicks: Heard one of their more famous songs in a restaurant the other night. They have completely disappeared. What happened? Did their protest of the Iraq War really kill their careers?
Elisabeth Shue: Saw the Saint was on television the other night. The 1997 film had Val Kilmer in it and she was probably at the height of her fame in the late 90s but I haven't seen her in anything in probably 20 years.