http://wiki.answers.com/Q/Has_an_NFL_player_ever_been_killed_during_a_game Answer
No player has ever been killed by a hit in the NFL.
There was one player, Chuck Hughes of the Detroit Lions, who suffered a heart attack and died during a game in 1971 against the Chicago Bears. His number (85) was retired and the Lions have an award named for him honoring the most improved player on the team.
There was another player, Reggie Brown of the Detroit Lions, who suffered a serious spinal cord injury in a game against the New York Jets in 1997. He was administered CPR on the field which saved his life. He had immediate surgery for his injury and his recovery was nothing short of miraculous. He now is in management of a car dealership group in Dallas and is able to walk and live as normally as anyone with that type of injury could hope.