Bug in the middle of a quest chain. What should I do? (I'm looking for help with the solution.)

I was doing a chain of missions in Felwood, for some reason that I don’t know, in the middle of the chain of missions I was instantiated to a dungeon and when I returned, now I can’t complete the mission, I abandoned it to resume it and now I can’t resume it either.

The mission in question is this:

I’m playing classic Cataclysm.