The recent Mac OS update to the Big Sur version has a commonly reported issue right now where certain programs may appear to not have any text. Some examples might be invisible text in spreadsheet cells or text in Word, or emails in a mail client showing the subject but not the message contents.
This appears to be a display issue with system fonts that is the culprit, so changing the default font in the program preferences or selecting a different font in programs like Office will most likely resolve it until the issue is patched. The most common reported fonts causing problems are Helvetica, Arial, or other Sans-Serif fonts with normal formatting (bold and italic seem to be fine).
Should changing the font displayed not work, others have had success in disabling font smoothing through Terminal with the command below:
defaults -currentHost write -g AppleFontSmoothing -int 0
The computer may need to be rebooted afterwards to take full effect.