Google ist Dein Freund
"Before you reset the PMU, try restarting your computer instead, which may resolve your issue. If your computer has stopped responding, try these steps, in order, until the computer responds:
Force Quit (Option-Command-Escape)
Restart (Control-Command-Power)
Force Shut Down (press the power button for 10 seconds)
If none of the above steps resolved the situation, reset the PMU. To reset the PMU on a Mac mini:
Unplug all cables from the computer, including the power cord.
Wait 10 seconds.
Plug in the power cord while simultaneously pressing and holding the power button on the back of the computer.
Let go of the power button.
Press the power button once more to start up your Mac mini.
Important: This procedure also resets the computer's PRAM. Be sure to reset your computer's time, date, and other system parameter settings as necessary."
zitat von apple.com