Can't add events to calendar app in Macbook Air

Two days after upgrading my MacBook Air to version 12.7.6, a host of problems occurred. I guess it’s appropriate to post separately about each one. 

The calendar:

1. I can’t add new events in the calendar app or in the calendar widget. I just get that spinning beach ball. 

2. All the information I had in the calendar app is gone. Is it possible to get that info back from a back up I made just a couple of days before upgrading? I’ve looked on my thumb drive using Time Machine but there’s no calendar app for that date. 

I've got a Macbook Air, Early 2015. 

Can anyone help?