Sounds like FM 11. Make a backup, then recover the backup and see if that's better.
It could also be a corrupted font on the computer, or the OS has been updated recently.
If still using FM 11 it's time to update to a later version. You could download a trial version of FM15, convert the file and see if that's better.
You can also duplicate the layout, select all on the layout and change the font to something different. If that produces readable text, a problem font file is a likely culprit.
google filemaker font issues
Yes! The problem has been solved. Several good suggestions were tried - plus variations. None solved the problem, The latest version of FileMaker, plus upgrading and trying several paths resulted in a full recovery. The data base contained 987 entries - a decade of information. Thanks!
This sounds like problems I experienced with version 13 on Win7.
Upgrading to Win10 helped the problem go away for me. I was very pervasive and persistent. These were on files on my computer shared with one co-worker peer to peer. She never saw it.