Better solution: from Layout Mode, select all text fields and change font to Arial. This seems to fix the problem for all records using this layout but does not however, explain what happened or how to prevent its recurrence.
You might check your fonts and font suitcases on your Mac. There's a long standing issue with macs where the presence of a duplicate or corrupted suitcase keeps FileMaker from displaying text formatted in that font.
If that's not the issue, you might run a recover on the file and see if Recover reports finding and correcting any issues with your file.
Thanks, will do.