Disappearing articles after 2.5 upgrade
From Joomla! Documentation
Problem: I have just upgraded to 2.5 from 1.7.3 and the only problem I have is that the articles that a specific user wrote are not being published. They still show up in the backend but not in the frontend. Other articles written by other users do show up.
Cause: If you have a user linked to a contact that is unpublished, the articles written by this user don't show up in the front.
Solution: Please check if you have a unpublished contact linked to user that wrote the articles. Also check the trashed contacts for this. The solution would be to publish the contact or to completely remove them, so empty the trash.