Team Monitor

With CTRL ALT DEL you invoke the Team Monitor which lists all currently running programs.

Programs that were launched by the system are blue, those started by the user black.
Applications that are unresponsive, which is often a sign the program has crashed, are marked red. You can try to quit a program by selecting it and pressing Quit application (or either DEL or Q). If that doesn't work, try Kill application (or SHIFT DEL or K) instead.

You can summon a Terminal with Open Terminal (or OPT ALT T).

Finally, if all hope is lost, you can Force reboot (or hold CTRL ALT DEL for 4 seconds).