A time-honored step for troubleshooting Macs. Done by holding Option+Command+P+R during startup.
From the Apple KB article:
If you experience issues related to settings stored in NVRAM, resetting NVRAM might help. NVRAM (nonvolatile random-access memory) is a small amount of memory that your Mac uses to store certain settings and access them quickly. Settings that can be stored in NVRAM include sound volume, display resolution, startup-disk selection, time zone, and recent kernel panic information. The settings stored in NVRAM depend on your Mac and the devices you're using with your Mac. If you experience issues related to these settings or others, resetting NVRAM might help. For example, if your Mac starts up from a disk other than the one selected in Startup Disk preferences, or a question mark icon briefly appears before your Mac starts up, you might need to reset NVRAM. Older Mac computers store similar information in PRAM (Parameter RAM) instead of NVRAM. The steps for resetting PRAM are the same as the steps for resetting NVRAM.