when an NPC dies,how i can make him appears again?
i need to speak with an NPC in a quest but he is dead,so i can't finish the quest

Well as far as I know, no. BUT!
if you are playing on the PC and open up the console, and type in ressurect on the selected dead target, it automaticly ressurects a character. Any information you got out of him/her you will have you get out of it again. Lets say you killed Maggie from Megaton, and then when you ressurected her, she will have been reset. Acting as she has never met you before EVER! Reseting the NPC's Memory.

Hope this helped.

I killed Arkansas early on and it turned out later that he would have been a useful option in a quest later. So if an NPC has a name it maybe think twice about killing him.

so i will have to restart the game.

thanks for the help.

You don't have to. If you look at your quest log, you will notice that Arkansas is not on the list anymore. Done.