>I can rebuild my project because a "Missing or invalid pjt" message which is not true because the pjt file is there, in fact it does exist!, ...unless is invalid but any way what that means??
>

Invalid means just that - there are formatting requirements on the information in the .PJT, and the file is failing in some fashion. It could be something innocuous, like an empty cluster at the end which is never referenced making the file length wrong, or it could be a subtle error that you aren't detecting yet. Your best course of action would be to rebuild the.PJT, or at least use it like a table and copy it,and rename it back after backing up, to see if that fixes the problem.