I am using Frame maker 2019 and lately I am unable to open any of the FM files on my machine, any insights on this will be much helpful.
What happens, when you try to open a file?
Can you open a book file?
Are the files local or on a network?
Can you create a new file with the FrameMaker default template, save this and open it again?
Does anyone else have access to these files or does anyone else edit these files?
When you open one of the files in a text editor, what is the very first line? (For FrameMaker 2020 this starts with: <MakerFile 16.0>)
Do you have the latest updated (Help | About)?
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.