Stuck in boss room after defeating the boss

Why is this still happening? I can't exit the boss room after I defeated the boss. Had this problem in forever now. Just lost one of the best map I ever had to this. I panicked and place a portal in the boss room. I think this screwed the chance of going back to the entrance. But player should not even go through that in the first place. after that, I tried to invite people to my party, join others. No one seems to know how to fix it. This is extremely frustrating! PLEASE FIX THIS ASAP!!!!