Well, first, make a backup, so you don't end up with worse problems. Of course, you could simply restore the file from a backup, if you have one (you do backup, of course).
Realize, that even if it is called ADDR.DB, it is NOT a database file. It is a plain text file. If you look in the Help, you will find a section that describes the data format (Address Book Data Format). Be sure to read the Note at the bottom of the page.
Now that you know the data layout, you should be able to open the file (in the text editor), and you will likely spot where something went wrong.
If you go to
http://www.os2voice.org/mailinglists.html you can subscribe to the PMMail/2 User mail list, when PMMail users hang out. That is the best place to get help with PMMail problems. It is pretty low volume.