If your Mac starts up to an Apple logo or progress bar
Your Mac shows an Apple logo when it finds your built-in or external startup disk. As startup continues, you should see a progress bar, with or without the Apple logo. This screen might alternate with a blank screen several times.
If you're installing a macOS update or upgrade
When you're updating or upgrading macOS, the Apple logo or progress bar might persist for much longer than usual. As installation continues, the progress bar might move slowly and pause for long periods. Apple recommends starting installation in the evening—so that it can complete overnight, if needed.
If your Mac is stuck on this screen or keeps restarting
If startup never progresses beyond an Apple logo or progress bar:
Press and hold the power button on your Mac for up to 10 seconds, until your Mac turns off. (Every Mac has a power button. On laptop computers that have Touch ID, press and hold Touch ID.)
Turn your Mac back on.
If the issue persists, press and hold the power button until your Mac turns off. Then unplug all nonessential accessories from your Mac, including printers, drives, and USB hubs. You could have an issue with one or more of those devices or their cables.
Turn your Mac back on.
If the issue persists, once again press and hold the power button until your Mac turns off.
Try to start up from macOS Recovery and repair your startup disk as described in this article: How to repair a Mac disk with Disk Utility.
If the issue persists, try to reinstall macOS from macOS Recovery.
If your Mac continues to start up to an Apple logo or progress bar, it might need service. Get service.
