In opening a small file (2 page letter) that was created two years ago, caused Framemaker 2019 to crash back to the desktop (I tried to attach the file but when I clicked 'Post' it got stripped out, saying the file doesn't match its file extension). Several other files from the same time period cause the same behavior. The file header says <MakerFile 12.0>
If there are any imported objects in the .fm document, try copying the file to some other directory, and see if it opens there.
You could also download a trial copy of FM2020 (perhaps on another machine), and see if that does the trick, and if so…
Not that it's of much help now, but I make a point, on wrapping up any document or project, of re-saving all the .fm files as .mif. This is mostly in case they need to be opened with a prior version, but it does provide a Plan B for when a newer version won't open it either. If the FM2020 trick works, save as MIF from it.