You are most probably the victim of a known bug. There is a solution however:
open your console and write simply:
-> this command should reset actors/characters to their default cell positions and Sharn should appear
IF she doesn't try this command:
PlaceAtPC "sharn gra-muzgob",1,20,1
This will create a new Sharn, so if she (the old one who disappeared) suddenly reappears, you'll have two of them.