How to Restart a Stuck iMac
Mac computers freeze for a variety of reasons. Software problems, viruses, spyware, system problems and hardware failures are just a few of the causes. You can try several keyboard commands to unfreeze the computer. Unfortunately, these methods don't always work, leaving the you forced to unplug the machine. This can cause damage to the hardware, so only do it if nothing works. You know your Mac is frozen if you cannot move the mouse, open any menus or the cursor has become a rainbow colored spinning ball.
Hold down the keys "Command," "Shift," "Option" and "Escape" to try to exit out of a frozen application.
Hold down the keys "Command," "Option," "Shift" and "Power" to turn off the computer.
Hold down the keys "Command," "Control" and "Power" to restart the computer.
Unplug the computer.