Winfried has given you some great places to start...but first, do you have a backup of the file?!
If her suggestions don't resolve the issue, try resetting your preferences by renaming the C:\Users\[username]\AppData\Roaming\Adobe\FrameMaker\16 directory to something like C:\Users\[username]\AppData\Roaming\Adobe\FrameMaker\16-old and restarting FrameMaker.
If that doesn't work, try someone else's instance of FrameMaker or download the Fm 2020 trial. If either of those options work, save the file as MIF and then reopen in your copy of FrameMaker.
If not, try importing the file (File > Import > File > Copy into Document) into a new FrameMaker document.