Press and hold the power button for about 10 seconds.
Press and release the power button, then immediately press and hold Command (⌘) and R until you see an Apple logo or other image.
Use Disk Utility to repair your startup disk.
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.
If none of these steps fix your Mac, then it’s time to take your Mac into an Apple store to let a professional troubleshoot your Mac.