My screen has froze, how can I unfreeze?
Please help me, my screen is frozen. Cannot open, close at all
iMac, Mac OS X (10.7.5)
Press and hold the power button until the screen turns black.
Then try starting up your Mac while holding down the Command + R kleys so you can access the built in utilities in OS X Recovery
From there you can repair, reformat and restore OS X.
You can restore from a Time Machine backup as well.
How to find the serial number of your Apple hardware product