If your Mac starts up to an Apple logo or progress bar
Your Mac will display an Apple logo when it finds your built-in or external startup disk. As the startup process continues, you should see a progress bar, with or without the Apple logo. This screen may 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 may persist for much longer than usual. As the installation process continues, the progress bar may move slowly and pause for long periods. Apple recommends starting the installation in the evening so it can be completed overnight if needed.
If your Mac is stuck on this screen or keeps restarting
If the startup process 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 non-essential accessories from your Mac, including printers, drives and USB hubs. There could be an issue with one or more of those devices or their cables.
Turn your Mac back on.
If the issue persists, press and hold the power button again 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.
