Which door do you mean? Past the library one of the corridors should be opened when you defeat Amdusias, and there is a teleporter shrine there, as well.
Once opened the door to Amdusias' chamber should remain that way. Do you have a save before fighting him that you can reload?
Did you quicksave/reload in the chamber where you fought Amdusias? Try exiting the game and reloading again (as long as you didn't quicksave inside with the door locked).
