Thank you for your post.
FileMaker Pro 12 needs to see specific information in the FileMaker Pro 11 file. Although FileMaker Pro 11 can still open the file, that doesn't mean it is free from damage. Launch FileMaker Pro 11, pull down the File menu and select Recover. Select the file and recover it. When the recover has finished, quit FileMaker Pro 11 and try converting the reccovered file to FileMaker Pro 12.
Please keep me updated with any progress.