so i'm having teh same problem as this person did. died. came back, the monster i did kill still gone, killed the rest of them. next door still locked. logged completely out and back in. still locked out. can not talk to npc. can not reset quest.
I think I know what your problem is, on one of the floors you have to fight the things without dying, if you die on that floor level then the doors are locked, if you don't die, they aren't locked and you can move on.