How to Roll back to a known stable version
If macOS won’t load, restart holding C+R after powering on and your Mac will start from its built-in Recovery partition. Many Macs since 2010 can fall back on Internet Recovery, indicated by a globe instead of the Apple logo.
Release the keys when one of those appears. When the macOS Utilities menu loads, you can reinstall the latest macOS that was on your Mac, check and repair the disk using Disk Utility, or restore from a Time Machine backup.
Revert to start
Hold Alt+Cmd+R at startup to upgrade to the latest compatible macOS. If you have Sierra 10.12.4 or later, press Shift+Alt+Cmd+R to reinstall your original macOS, or the closest available — great for rolling back to what works.