what so u do if a person dies and there part of a quest and you didnt know they were when they died and there body has disappeared?

On PC? If so, you can use a cheat code to bring the NPC back. If you're playing on a console, I don't know. someone who knows something about consoles will have to answer that. I'm guessing you'll have to go back to an earlier save.

I'm playing on a pc. I dont have a save file that goes that far but was wondering if theres a cheat or something so that I can clone him bak?

If he was a very important quest character, he wouldn't die. If he's just some ordinary guy, than he might be dead, but he mustn't be that important to begin with. I say, cut your losses and just move on.

But since you're on PC, there are commands which can bring him back.

Here's the Wiki page listing Oblivion console commands: http://www.uesp.net/wiki/Oblivion:Console#resurrect

I don't use the console except COC for testing purposes so I have no personal experience with this kind of thing. Somebody else nay know, though. Good luck!

thanks for all your help. he wasnt an important character I think but u need him to start a thieves guild quest

I'd suggest looking the specific character up on the wiki. There will be a link to the quest page for any he's involved in, and there's usually a note about what to do if someone dies.

A few of the quests are allowed to fail if a related character is killed, but IIRC they're all side-quests and not part of the Main or Guild lines. An example is Ulrich Leland. He can die at the battle of Bruma and prevent that quest from being completed. I believe the other two characters in the same quest can also get killed prematurely.

The only non-essential NPCs that figures in a Thieves' Guild quest are Ontus Vanin, and that quest can proceed without him, even though it doesn't make so much sense that way, and the house owners for the locations where you meet the Grey Fox, who don't need to be alive either.