I have checked the documentation of the software and I couldn't find anything for password reset. After that, I checked the forums on the SourceForge website where PNotes is hosted.
It seems only the developer knows how to reset the password while keeping your notes intact.
Use the following link to post on the developer's forums: http://sourceforge.net/p/pnotes/discussion/762673/?page=1
Noe: You might need to create an account in order to post.