After installing update KB5003173 (2021-05 Cumulative Update for Windows 10 Version 20H2 for x64-based Systems), MS 365 mail no longer worked. I could see message headers in various folders, but was unable to display the message body. Calendar entries were also hosed.
The 'fix' for me was to do an online repair, which removes/reinstalls the app.
Display the Task Bar
Type Apps & Features in the Search Box
Under 'Best match', click 'Apps & Features'
Scroll down to find "Microsoft 365" (Note: choose the version you're currently using
Click 'Modify'- Yes
First try 'Quick Repair' (didn't work for me)
If no joy, repeat the above steps, but choose 'Online Repair', which removes/reinstalls MS 365 (DID work for me)
Reboot and enjoy (note: I had to reboot twice, then it functioned normally.)