Are you sure she didn't just wander over beside a wall, or something? Check your mini map as you search the place. You didn't drink too much of Rimmer's ale did you, and stumble into the wrong house? <img src="/ubbthreads/images/graemlins/biggrin.gif" alt="" /> She shouldn't leave, doesn't go downstairs until you give her the items for the summoning quest, and there are no hostile creatures around after you save her from the first demon she tries to summon.
You can use part of the procedure in
Reviving dead NPCs to re-set her position, if you can not find her.