this thread should be moved I think, either to Cheats, Hints & Spoilers or the troubleshooting forum. It might garner faster attention there.
I'm forgetting which quest this is. If it's the one I'm thinking of, there's some weapons & armor you have to retrieve? Sorry it's been awhile. I'm not good with quest names, anyways. Anyways, I think it's the one where
Spoiler there's some weapons & armor in this cave or something, which belonged to this fallen knight or something?
If that's it, there's not much you can do in vanilla, but possibly the console might make a difference. Others here should be able to help you better than I can.