where did my calendar go and how can I get it back? And why did an old calendar long-ago unused become my new default calendar?

Had a recent update (26.2.1 I believe). Don't know exact date of the update, but do know that now my iphone calendar is completely missing, replaced by some generic calendar. I've done the "toggle" thing that some have suggested for clearing a synch issue. I've tried restoring from an archived icloud backup of the calendar, but the archive only goes back to yesterday. Doesn't help. The update was from several days ago.

Why would my calendar just be deleted? Also, my "default" calendar is a calendar with events that I created probably 15 years ago that were deleted events several years ago (probably around 10 years ago or so) and those events are now appearing in the "default" calendar. (Events like my ex-wife's birthday, birthday of a co-worker I had 3 jobs ago, etc) I really don't understand this. Can someone try to explain what has happened and how can I fix this and get my calendar that I had just a week ago back?