If your Mac displays a grey screen at startup
If you experienced a blank grey screen after starting up your Mac, there may be a problem with a firmware update. If you experienced a grey screen with an Apple logo (with or without a gear icon) and a flashing question mark, there may be a problem with missing, incompatible or damaged software.
Note: To print these instructions, press Command-P.
Make sure the latest firmware for your Mac is installed. You can check the Apple Downloads website for the latest versions.
If you just installed a new version of macOS, make sure your Mac has the recommended memory and disk space. To check, 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.
Try using macOS Recovery. It can help you restore from a Time Machine backup, reinstall macOS, get help online, repair or erase a hard disk and more. See the Apple Support article About macOS Recovery.