Disconnect your computer from your internet connection.
From the Menu, select Force Quit...>Calendar.
Paste this text ~/Library into the Finder>Go>Go to Folder...window and select "Go." This will open the Users Library Folder.
Then open the Calendars Folder in that Library Folder, and move any any files with "Cache" listed in the name field to your Desktop.
Next open the Preferences Folder in the same Library Folder, and move the com.apple.iCal.plist file to the Desktop.
Log out/in or restart, and open Calendar.
If you found the Users Library Folder using the directions that were previously posted, you should find a "Preferences" Folder in that Library.
Can you view your calendars on iCloud.com?
If your events are listed there, go to your iMac>Calendar>Preferences...>Accounts>Account Information...and uncheck/check the "Enable this account" selection: