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If it is frozen or not working, then you need to force quit it.
Press Command, Option and Esc keys and this will bring Force Quit. Select Photos and click Force Quit. Then shutdown.
If that didn't work, then you need to force turn off your Mac. Press and hold down the Power button of your Mac for 5-10 seconds and it will turn off. Then turn it back on.
Please let me know if that didn't work, so I will continue to help you