1. Shut down your machine. Yes, all the way down, not sleep or logging out 🙂
  2. Press the power button and then press command-option-p-r immediately…
  3. Release the keys after you hear three startup chimes and then your Mac will reboot normally.

What Is PRAM (Parameter RAM)?

PRAM (Parameter RAM) is the memory that stores default settings for the nitty gritty details of your Mac. It contains stuff like the alarm clock setting, mouse scaling, start up disk, virtual memory and speaker volume among other things. Resetting the core data to its default is a great way to restore things to way they are supposed to be.