This should be in the Hints and Spoilers section.
I'm afraid there's nothing you can do except...
Spoiler ...wait for it to respawn. I believe that's a maximum of 10 game-days now.
These sort of radiant quests often suffer from this problem, and sometimes it's worse:
Spoiler I had a radiant quest from Farkas to rescue a kidnapping victim. Fine. Went there, killed the baddies, quest updates with, "Speak to ." Kind of hard to do since it was a corpse. That particular NPC was killed in another quest halfway across Skyrim a long time ago. Lucky for me I'm on a PC and could fix it.
The only advice is, if you are on a console, save before picking up these radiant quests.