Press command + option + esc keys together at the same time.
When Force Quit window appears, select the application, if not already.
Press Force Quit button at the bottom of the window.
Force Shut Down
Press the power button for 10 seconds or more.
Computer should shut down.
OS X 10.7 /10.8
Steps 1 through 7
If this does not help, contact Apple.
I see this happening several times and especially since the latest OS X (10.9) updates.
Usually it happens when starting work after sleep mode. I just open a newspaper webpage (even refresh) scroll down a bit and it freezes.
I would like to know if there are others seeing the same behaving?
Thanks and regards,