If you see a gray screen at startup
If you see a blank gray screen after starting up your Mac, you may have a problem with a firmware update. If you see a gray screen with an Apple logo (with or without a gear icon) and a flashing question mark, you may have a problem with missing, incompatible, or damaged software.
Note: To print these instructions before you start, click the Share button in the Help window, then choose Print.
Make sure the latest firmware for your Mac is installed. Choose Apple menu > App Store, then click Updates or check the Apple Downloads website for the latest versions. If you recently installed a new version of your computer’s firmware, try installing it again.
If you just installed a new version of macOS, make sure your Mac has the recommended memory and disk space. To check your computer’s specifications, choose Apple menu > About This Mac, or refer to the printed instructions that came with your Mac.
If your Mac doesn’t meet these specifications, reinstall your previous version of the operating system.
Use the Disk Utility app on your Mac to repair your startup disk.
If you’re starting up from a DVD, it must be a disc that came with your Mac. Discs from other Mac models may not work with your Mac.
You can also use macOS Recovery, which helps you restore from a Time Machine backup, reinstall macOS, get help online, repair or erase a hard disk, and more. For more information, see the Apple Support article About macOS Recovery.