Follow these steps for iMac, Mac mini, Mac Pro, and Xserve.

Choose Apple menu > Shut Down.

After your Mac shuts down, unplug the power cord.

Wait at least 15 seconds.

Plug the power cord back in.

Wait 5 seconds, then press the power button again to turn on your Mac.

For Intel-based Xserve computers that aren't responding, you can shut down locally or by using remote commands. You can also press and hold the power button for 5 seconds.