Outlook .pst file password

When one of our users came back from taking a vacation, she tried to check her email by opening Outlook (which is normal). However, for some strange reason it’s asking for a password to access her personal folders (outlook.pst). Why is it asking for a password? We never gave it a password! I have even tried four different utilities to try to restore it, but all of them indicate that there is no password on the file. We are using Outlook 2002. How do we get into this file or recover it? It’s not like we can just create a new one, because she has a lot of very important emails, contacts, etc.