Help! I'm at the end of KOTN, the Umaril quest. I've arrived with my knights in the final dungeon, entered the first big room and killed the two enemies. I hit the button on the altar, but nothing happens -- the gate on the ground floor doesn't open, and the two gates up the stairs don't open. Thus, I'm stuck.

I have no clue what to do, and am dying of frustration -- I tried clearing the cache and reinstalling the expansion pack, I've tried going back on old auto saves, I've tried going back outside, etc.

Help! Thanks!

I should add that I'm playing the game on an xbox 360.