Try using recover to recover the file and then see if the recovered copy will open.
Assume it is starting on the server and having status "Open"?
Do you have a startscript?
1: If you have FileMaker Pro Advanced then please try opening with the debugger active and follow the process.
2: Try closing the file on the server and then open again ... does it open without error messages?
3: Try to set the server backup schedule to verify the file after backup ... any messages?
3: Consider taking a copy, open it locally on your PC/Mac in Pro (not for live use, just for testing) with the debugger open ... what is the result now?
Do you have a backup from before the problem occurred?
Is it being opened directly, or as an external data link? If the latter, check that the link is pointing at the right place—reset if necessary.