Force-quit a stuck app by pressing ⌘-Option-Shift-Escape. Try it once and see. You have to hold the keys down for a few seconds but it will work eventually, pulling the plug on the app you’re using. This is not the way you want to quit ordinarily, but sometimes things get stuck. ⌘-Option-Shift-Escape is a great way to ditch the app that’s stuck.
Want some more, longer how-tos?
Visit my other website, christianboyce.com