Q: the screen is frozen ???

Press command + option + esc keys together at the same time.

Wait.

When Force Quit window appears, select the application if not already.

Press Force Quit button at the bottom of the window.

Wait.

Application will quit.

If this does not help, press the power button for 7 or more seconds.

Computer will shut down.

Restart.

http://support.apple.com/kb/HT3411