For example, if your Mac starts up from a startup disk other than the one you've specified in Startup Disk preferences, or if a "question mark" icon appears briefly when your Mac starts up, resetting NVRAM or PRAM may help.
• Shut down the computer.
• Locate the following keys on the keyboard: Command, Option (Alt), P and R.
• Turn on the computer.
• Press and hold the Command-Option-P-R keys (you must press this key combination before the gray screen appears).
• Hold the keys down until the computer restarts and you hear the startup sound for the second time, then release the keys.
After resetting NVRAM or PRAM you may need to reconfigure your settings for speaker volume, screen resolution, startup disk selection, time zone information. If issues persist, your Mac's logic board battery (not a portable Mac's rechargeable battery) may need to be replaced. The logic board battery helps retain NVRAM/PRAM settings when your computer is shut down. You can take your Mac to a Mac Genius or Apple Authorized Service Provider to replace the battery on the logic board.
In some situations, you may need to reset your computer's System Management Controller (SMC). Learn how to identify these conditions and reset the SMC.