Q: The calendars on my iPhone and Ipad have suddenly stopped syncing with Outlook on my PC

Hello there Treachem!

Making sure that iCloud Contacts and Calendars are syncing properly is certainly a must since they're something that we use on a daily basis.  Let's take a look so that we can make sure that we get them back to showing correctly on all of your devices.  Take a look at this article since it goes through many different steps to do on your iOS devices and on your PC: 

Get help using iCloud Contacts, Calendars, or Reminders

Have a great day!

Thank you for trying those steps!  Since it still isn't allowing you to select it, the best thing to do at this point would be to contact Apple Support.  That way the issue can be looked into further.  Here is the link for that:

Contact Support

Cheers!