You didn't state if you were using version 12 or 13. The file will open in 12 but will not open in 13. I ran recover (in 12 and 13) on the file and it didn't detect any errors. I assume that there is some kind of layout corruption that 12 doesn't detect or ignores because it pertains to a new feature in 13. I would suggest using a backup copy.
Tech support monitors this forum but It is suggested to use the "Report an Issue" at the top of this page to report a bug or strange behavior such as this. Tech support may need to look at the file to determine the issue with it.
I am using 13, over the weekend managed to copy the tables into a new database and started again with the forms a scripts improving some areas so I am quite happy with that, but I will report this issue as I have lost a bit of confidence in Filemaker and wouldn't want this to happen to any other files.