Quit the Calendar application.
If the calendar cache of the macOS Calendar application gets corrupted, it may cause all sorts of weird behavior. Typically calendars have stopped synching to/from iCloud, or some calendars are not shown at all (although the events in the may still show).
This can be resolved by clearing the calendar cache. When the macOS Calendar application is reopened, it will regenerate this cache.
Select File Duplicate to make a backup of the folder, just in case something goes wrong.
Do the following only if something goes wrong later in this process. You can restore the backup by moving the folder Calendars elsewhere, and renaming the duplicate Calendars copy back to Calendars.