Natasha Yar was killed off in season 1, although she does come back in an alternate timeline in 1 episode, and in the past in another episode. Data was killed off after the series ended in Star Trek:Nemesis. Data was resurrected in the graphic novel: Countdown.
Real world: Denise Crosby left Star Trek: The Next Generation, during season 1, because she thought Trek was uncool. Real stupid. Apparently even Crosby thought so, because after leaving Trek she did a nude photo shoot for Playboy. Everybody thought her tits were too small and she couldn't get work as a result. She begged to be brought back and appeared as the daughter of Yar from a different timeline in several subsequent episodes. ProfessorTrek 01:56, July 1, 2011 (UTC)