Try recovering the file.
Launch Filemaker without trying to open your file. Select Recover from your File menu and select the file you want to recover. This process creates a new "recovered" copy of the file. Try opening this copy and see if you can.
If this allows you to gain access, I strongly recommend that you replace your recovered copy with a back up copy if at all possible. The recover process' main priority is fixing damage to the file so that you can access the data. That way you can export the data into an undamaged back up copy. Other problems with the file may persist after it has been recovered.