Reset your PRAM
- Remove all external devices connected to your MacBook, example, speakers, external screen, keyboards.
- Shut down the computer.
- Find the keys on the keyboard: Command, Option, P, and R. You will need to press and hold these keys down simultaneously and Turn on the computer.
- You have to make sure you get those keys pressed before the gray screen comes up or it won’t work.
- Hold the keys down until the computer restarts and you hear the startup chime.
- Release the keys and let your Mac boot normally.
Your computer’s PRAM and the NVRAM are reset to the default values. If resetting PRAM does not resolve an issue, resetting the PMU may be the next appropriate step.