I'm unable to find what PNO is, but I"m guessing that it's a missing Place Number or order or something. Anyway, if you make sure you have a backup of your project somewhere, I'd suggest that you choose Project from the Utilities menu and then Check. Select all options in the Project group and then click Repair. Any places used in the current project that are missing should be restored. If you receive "Missing Place no.nn" in any of your records, tab to that field and overwrite the Missing Place with the correct name and press F9. You will get a message that asks if you want to keep this name (or something like that) and click Yes to change to the correct name you just entered.
Now, after I say all of this ... if PNO is something else (and I doubt it is), I'm sure someone more knowledgeable will pipe up. :)