1. Backup your save file, and then reinstall the game.
If that doesn't fix the problem, then:
2a. If it is a random or guild quest: I'd suggest you just ditch the quest and try a different one. Failing quests is no big deal; you can always take another. DF quests are like wives in that respect.
2b. If it is a main storyline quest, well. That svcks then
but no real reason to start over. You don't have to do the main quest to enjoy the game. I played the game for 11 years before ever attempting the main storyline. And I'm awesome, so being like me is ok.
Also: repeat this Daggerfall mantra:
Save Often
Save before Quests
Use Multiple SlotsSeriously, keeping a Main and Quest slot for each character is pretty smart.
edit2: Oh yeah. And make sure you're patched to version 1.07.213