Skyrim main quest line broken?

[Arjahn]

I've been playing through The Elder Scrolls V: Skyrim, and the game runs fine until I get to the old man in the prison cell (sorry, forgot his name) as part of the main quest. It's supposed to go that you talk to him and he opens the door, but after I talk to him the door just stays closed, and if I try to noclip into his room I can't talk to him. Also, if I stab him, he attacks me (the bastard). I've tried reinstalling the game and still, nothing happens.