If you have updated your MacOS or done something really wrong and you cannot boot into your MacOS anymore, you can perform a recovery from Time Machine.
NOTE :
You need to make a backup of the MacOS via Time Machine BEFORE you make any changes.
- Boot into MacOS Recovery partition.
- Choose Recovery from Time Machine.
- Agree that it will erase everything on the destination drive.
- Select the latest Time Machine backup of the MacOS.
- Wait for the recovery (usually takes hours!)
I have tested this method and it can be relied on in confidence!