Q: Calendar has stopped syncing with iCloud.

Using OS X 10.10.3, Calendar has stopped syncing with iCloud - it advises me to check the internet connection. All other apps work fine and Calendar syncs with my IOS devices. Mail syncs across OS X and IOS with no problems. 

The error message - Your calendar couldn’t be refreshed. - says it cannot sync with cal.me.com (my default account has always been @me.com) and I cannot see any way of changing this - norman I explain why it stopped working with the last OS X upgrade.

Any ideas?