Teleporting to the end

I tried to teleport another player (from normal world) to the end (I was in the end) using /tp command and it said something like "An error occured during this command" so I died and teleported back to the normal world and ender dragon from end was there, so I tried to kill it, but when ender dragon HP reached 0 it only stared exploding but nothing else. I saved world and loaded again but ender dragon was still there and cannot be killed.