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If iPhoto is frozen, then you can Force Quit it. Press Command(Apple) + Option + Esc
keys and this will bring the Force Quit window. From the Force Quit window, select iPhoto and click Force Quit button.
If that didn't work, then press and hold down the Power button of your Mac for 10 seconds and your Mac will force-turn off. Turn it back on..
Please let me know if that didn't work, so we can continue troubleshooting. Don't Rate my answer until you are satisfied and solved the problem.