A character who has chosen a patron deity can be brought back from the dead by all the normal methods, provided the character is willing to return. The process is somewhat more difficult for a character who did not choose a patron deity in life.
Kelemvor, the god of the dead, eventually disposes of unclaimed souls, trapping them and making it impossible for them to return to life. This process of disposal takes 1d10 days. During this time, characters who died without a patron deity can be raised, resurrected, or reincarnated if the spell that brings them back is cast before Kelemvor deals with the soul. After this time, only a miracle or wish can restore the character to life.
Even if a player has not chosen a patron deity for his character before the character meets her death, the player can choose one at the time of the character’s death. If the player decides not to choose a patron once his character has died, the character is truly faithless and must take his chances with the rest of the unclaimed souls of the Fugue Plane. If the player decides to declare a patron, he should choose a deity the character has shown at least some interest in. Even if the character has never actively shown interest in any particular god, the way the character has been played usually will suggest a god. For example, a character of good alignment who has devoted himself to magic would naturally gravitate toward Mystra, whereas an adventurer of almost any class or alignment might naturally gravitate to Shaundakul if he had a zest for travel or exploration.
In short, there is one cardinal rule regarding characters and patron deities: Never punish a player for not writing down a patron deity on his character sheet.