Create a back-up and run the repair utility (for Pro 5.0 this fpautl32.exe, for all new versions this is called repair.exe).
This error means there's a half-written record and the database is flagged as unrecognizable or corrupted or something else happen like the database was overwritten by another file or the database resides on a corrupted or failing hard drive.
NEVER run the FPASYNC.EXE utility to fix this error!
If repair.exe does not fix the error, then the database is corrupted beyond repair and must be restored from a backup. The computer should be check for potential hard drive or network problems.