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We can force quit the Calendar. Press Command, Option and Esc keys at same time and this will bring Force Quit. Select Calendar app and click Force Quit. Then open it.
Please let me know if that didn't work, so we can continue troubleshooting.
If you are unable to force quit Calendar, they try this. Press and hold down the Power button of your Mac for 5-10 seconds and this will force turn off your Mac. Turn it back normally. Please let me know if that didn't work, so we can continue troubleshooting.