Try these in order. Each one goes a step further. Anything unsaved in a closed app is lost.
- Press Ctrl + Shift + Esc, as on Windows. System Monitor opens. Click Applications on the left, click the frozen app, then Quit Application, and Quit Application again to confirm.
- Still stuck? Press Windows + Ctrl + Esc. The pointer changes. Click the frozen window and it closes. Esc cancels.
- The whole screen is stuck: press Ctrl + Alt + Del, then choose Restart.
- Nothing responds at all: hold the power button for about ten seconds until the computer turns off, then turn it on again.