What operating system are you on? Have you recently moved server?
What version of fCraft are you using? (latest is 0.616)
Have you changed any fCraft configuration, edited PlayerDB manually, installed any new software, or changed any OS settings right before this started to happen? Any other notable things you did before this happened?
If you can zip and email a copy of your PlayerDB to [email protected], I'll see if I can recover it.