Party killed a boss but i died. Now it isn't there and i can't kill it and carry on.

Me and my party went to kill the brute in the prison. We started it and i died, they killed it however. I didn't complete the quest though and now i can't complete it. Everytime i try to go to the boss he isn't there and i can't continue. Any help?

Control click the entrance to the zone (Upper Prison I believe) to create a new instance, thus resetting the boss. And if you stay dead in the area while the boss dies, you will get quest credit for it.