Summarize this article in one sentence.
Double-click the calendar icon in your Mac's Dock. If you have multiple calendars synced to different accounts, you'll need to back those up, too. To do so:  Click File  Select Export  Click Calendar Archive  Select a save location. You may be able to solve your calendar problem by removing the calendar account from your computer and then adding it back. To do so:  Click the Apple Menu {"smallUrl":"https:\/\/www.wikihow.com\/images\/f\/ff\/Macapple1.png","bigUrl":"\/images\/thumb\/f\/ff\/Macapple1.png\/29px-Macapple1.png","smallWidth":460,"smallHeight":476,"bigWidth":"29","bigHeight":"30","licensing":"<div class=\"mw-parser-output\"><p>I edited this screenshot of a Mac icon.\n<\/p><p>License: <a rel=\"nofollow\" class=\"external text\" href=\"http:\/\/en.wikipedia.org\/wiki\/Fair_use\">Fair Use<\/a><br>\n<\/p><\/div>"}  Click System Preferences  Click Internet Accounts  Select the calendar's account. Click -  Restart your Mac Go back to the Internet Accounts page. Select an account provider (e.g., Outlook). Enter your account's email address and password. If the problem was fixed, your account simply needed a refresh. If not, continue to the next step. There is a decent chance that your calendars' contents will be completely deleted if you clear your cache, so make absolutely sure that you have a backup file for each of your calendars. Only proceed after you are certain that you won't lose anything important. You are now ready to proceed with clearing your calendar's cache.

Summary:
Open the Calendar app. Back up your calendar's data. Try removing and re-adding the account. Close and re-open your Calendar. Double-check your backups before clearing your cache. Close your Calendar app.