In the upper room you'll find a secret door. Check your map to see it. Down a poorly textured passage you'll find a floating skull. That skull will teleport you to the rest of the dungeon. Oh joy, a teleporter dungeon; hope you have a Recall spell, or it may be rather frustrating!
And by the way, the part of Ysabeth that you'll find isn't very big, so keep your eyes open.
[edit] I really wish people would stop pointing others to the cheats the instant there is trouble. Just because someone can't find something, or doesn't know what to look for, doesn't mean the game is bugged. The whole "quest target is in an unconnected room" is rather uncommon.
Oh, and I'm not directing that wholly at you, MGF
But in general, a lot of people seem to do this.