Using older versions of Outlook for Mac for personal email and calendar
Assuming people are still experiencing this (as I just did), it's not necessary to quit the Calendar app or force quit every process. If you launch the Activity Monitor, look for "CalendarAgent". Just quit it (no need for force quit), and it restarts immediately, restoring all the missing items. I've done this in the past and it worked just now again, so I think it's worth trying.