Screen frozen

According to Apple:

Your Mac shows an Apple logo when it finds your local startup disk, then shows a progress bar as the macOS startup or installation process continues.

Your Mac shows an Apple logo when it finds your startup disk, which is usually the disk built into your Mac. 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 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. That's why Apple recommends beginning macOS installation in the evening—so that it can complete overnight, if needed.

If your Mac is stuck on this screen

If you think you've waited long enough to know that your Mac is stuck on this screen, follow these steps.

1. 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 notebook computers that have Touch ID, press and hold Touch ID.)
2. Turn your Mac back on. If the issue persists, press and hold the power button until your Mac turns off. Then unplug all accessories from your Mac, including printers, drives, USB hubs, and other nonessential devices. You could have an issue with one or more of those devices or their cables. 
3. Turn your Mac back on. If the issue persists, once again press and hold the power button until your Mac turns off. Then follow the steps in this article: How to repair a Mac disk with Disk Utility. It explains how to start up from macOS Recovery so that you can use Disk Utility.

• • If Disk Utility found no errors, reinstall macOS.
  • If Disk Utility found errors and repaired them, restart your Mac. If the issue returns after restarting, reinstall macOS.

Need more help?

• Call Apple Support at 1-800-APLCARE. 📞
• International Support. 🌐
• Contact Apple Support. 📝
• Chat using Messages. 💬