This exact same thing happened to me too.
Very sad, because I was never able to finish the fighters guild with that character. Don't worry though, this won't happen to other characters you may create/have already created. I myslef created three others since then and the body was right on the quest marker for all three.
I feel your pain, I had logged over 100 hours with my character.
This is a known bug and has been discussed here on this forum in the past. Go into the command console by hitting the Tilde "~" key and then type in this command exactly as shown:
player.setstage fgc10swamp 40
This will advance the quest and let you finish it. If it doesn't work, then you have not typed it in exactly as shown with spaces etc. so keep trying until it works. Good luck.