Sounds like the creator code of the file was changed. Do you have a copy of that venerable utility known as ResEdit? If so, you can check the creator code of the file that works and then edit the file where this changed to match.
You may also be able to launch FileMaker and then use Open from the file menu to open the file without resorting to ResEdit.
Oh yeah, make sure you keep several back up copies and not just on the flash drive! That allows you the option of reverting back to an undamaged copy should this happen again.