An old example would be McLean Stevenson from M*A*S*H; worst mistake he ever made. Or the woman who played Kate in NCIS; I haven't heard much from her, save for that stupid "Rizzolie and Isles" show. Has anybody seen the guy who played Sweets in "Bones" lately?
Oh here's a good one: everyone who ever left the "Dr. Who" shows: you never see or hear from them again except in tv productions that are mostly ignored or are very forgettable.
David Tennant? Christopher Eccleston had a solid career before and after Who. Freema Agyeman seems to be doing pretty ok for herself. Peter Capaldi had such a strong career before I'm sure he'll be fine as well. Matt Smith had the Crown which really wasn't ignored.