Ursprünglicher Beitrag von: Alfy ,
Turn it off completely. Then press and hold the power button till you hear the chime sound. Turn off completely again. This time press the power button nd right after you press it, press me hold the "command+option+p+r" keys. You should hear it chime then reboot and chime again. Hopefully this fixes it. Last option you can do is do a SMc reset.