I can not see my calender at all - what have I done? and how can I fix it?
Did you by any chance hide the appropriate panel on the left side?
If so, you can get it back with using the context menu when clicking e.g. on the “Mail” panel and put a mark on the Calendar entry.
If not, we will need some more information. E.g where do you miss the calendars? GMail, Outlook, local ones or those with CALDAV protocol, Exchange… Did you change anything in your configuration, added or removed any accounts?
Perhaps you’ve just unchecked one or more of the calenders?