If your Mac starts up to a blank screen
A blank screen (black, gray, or blue) should appear once or more as your Mac starts up. If your Mac appears to be stuck on a blank screen, follow these steps.
Press and hold the power button on your Mac for about 10 seconds.* If your Mac is turned on, this turns it off.
Press and release the power button and see if your Mac starts up.
If your screen remains blank, press and hold the power button for about 10 seconds again to make sure that your Mac is turned off.
Try to start up from macOS Recovery, then use Disk Utility in macOS Recovery to repair your startup disk. For details, see How to repair a Mac disk with Disk Utility.
If you can't start up from macOS Recovery, your Mac might not be turning on at all. In that case, follow the steps for if your Mac doesn't turn on.
If you're using an external display instead of a built-in display, make sure that the issue isn't with your external display.
If the issue continues or you need help with any of these instructions, contact Apple Support.
* Every Mac has a power button. On Mac laptop computers, the power button is in the upper-right corner of the keyboard. If your laptop keyboard has Touch ID, that key is also the power button.