Calendar Missing from Account – Can't Add via File > New Calendar, Calendar Deleted

My Calendar displays all my different account calendar in the app, but I’m unable to add under a specific account as is doesn't show up under File --> New Calendar, and it just deleted an entire Calender from that account.

I've tried to restart my laptop and the app by itself. However, it just won't appear.

Thanks for any help in advance!

[Edited by Moderator]