Shut down your Mac. Unplug from power supply. Leave it unplugged for 20 minutes or more. (I had always heard just a few minutes would drain but Apple Support advised me leave unplugged for longer.)Unplug all your devices but your keyboard and mouse.Zap your PRAM 1. Shut down your Mac. 2. Locate the following keys on the keyboard: Command (⌘), Option, P, and R. You will need to hold these keys down