Missing User

I have a user who is registered on my wordpress site but does not appear in the list of users so I cannot update her capabilities. I use a SimplePress Forum and she is listed on the users there, but as forum users have to be wordpress users first it seems odd that she does not appear as a wordpress user. I have tried adding her back in but just get a message to say that the user already exists, however when I try to run a search for her I cannot find her and she does not appear in the list. Any ideas???