If your Mac won’t boot, there’s a hidden “Recovery Mode” that you can use to diagnose and fix issues or reinstall macOS completely.
The first thing you’ll need to do is boot into Recovery Mode. Fortunately, that’s easy. Turn off your Mac and then restart it, holding down Command+R while it starts. This should get you into Recovery Mode, where you’ll see the macOS Utilities window. Recovery Mode provides four services. You can restore from a Time Machine Backup, get help online from the Apple Support site, try to fix disk problems with Disk Utility, and reinstall macOS. We’re going to be focusing on those last two options in this article.
RELATED: How to Fully Restore macOS From a Time Machine Backup in Recovery Mode
Run First Aid From Your Recovery Partition
The Recovery Partition on all Macs comes loaded with Disk Utility, which can run “First Aid” on a drive that may be corrupted, and attempt to fix some of the issues. This
Warning: If the Disk Utility tells you that your drive is about to fail, take it seriously. We recommend backing up your Mac (if you’re not already doing it) and replacing the disk. Disk Utility won’t be able to fix a failing disk.
After it’s done, try booting into macOS again. If Disk Utility wasn’t able to fix all your problems, you could try running it again. If often can fix more on the second run-through.
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Source : How to Fix Mac Startup Problems Using Recovery Mode
