NVRAM (nonvolatile random-access memory) is a small amount of memory that your Mac uses to store certain settings and access them quickly. PRAM (Parameter RAM) stores similar information, and the steps for resetting NVRAM and PRAM are the same.
- Shut down your Mac.
- Turn it back on. Press and hold Option, Command, P, and R.
- Release these keys after 20-25 seconds.
Reference: Apple Support HT204063