If you press Ctrl+Alt+Delete, Ubuntu brings you a menu to shut down, restart, or suspend your system. But for some reason you might encounter that the system freezes, the mouse cursor can’t move, neither pressing Ctrl+Alt+Delete can work.
Remember that there’s a shortcut key Alt+PrintScreen+K that can bring you back to the log-in screen immediately without the need to reboot the system. That’s a time saver.
As an alternative, you can also use Ctrl+Alt+Backspace to do the same after you’ve enabled the shortcut key by the following steps:
Go to System > Preferences > Keyboard.
Select the “Layouts” tab and click the “Options” button.
Select “Key sequence to kill the X server” and enable “Control + Alt + Backspace”.