Calendar disappeared on iMac (but still available on iCloud) - cannot retrieve it on iMac

All my calendars have disappeared on my iMac

The calendar is completely blank

I share multiple calendars and they have all gone too

The calendars are all still there in iCloud

Here's what I have tried so far...

• Turning calendar off and on via settings

• Turning calendar/reminders off in settings, leaving it a few minutes, turning it back on and then restarting the iMac (actually tried a number of different variations on this one) - the reminders disappear and come back but the calendar is still not populating

I can access the calendars on my phone and also using the iCloud calendar on a browser

What should I try next?

One point to add, I'm still on Catalina as my machine is now too old to upgrade to Big Sur - could it have something to do with this?

Thanks in anticipation of any help you can offer

All the best