Can a dead player have life restored later to bring them back into the game?
A player at 0 life loses the game permanently. Everything they own (including e.g. permanents that are under someone else's control) vanishes from the game; neither the player nor their stuff is a legal target anymore.
From the Comprehensive Rules:

104.3b If a player’s life total is 0 or less, he or she loses the game the next time a player would receive priority.
104.5. If a player loses the game, he or she leaves the game.

If you want more specifics on what happens when a player leaves the game, see rule 800.4.