If TntMPD is trying to reopen your most recent database, but for some reason that database is not available, you might get this error. If you're running TntSync and trying to synchronize two TntMPD databases you might also get this error.
It usually means that the database was on a remote computer or on a removable drive. Make sure that the remote computer is turned on or that the removable drive is plugged back in. If that doesn't work, open up Windows Explorer or Mac Finder and manually see if the file reported as missing truly is.