What happened to my MacOS? After I put my MacBook to sleep, like I always do; the following day,  I open the lid and it’s off for some reason. I turn it on and it powers up. I select my user account and enter the password. It loads and then freezes/hangs about half way through. It then reboots and loads some weird screen saying and macOS installation failed either restart, shut down, show log/save log. 

What the is that exactly? Did my OS got corrupted for some reason? What should I do now?
If you can boot to Recovery HD,  then you can check for a local Time Machine back up or external drive backup and make a decision. 


Data is most important, so take the Mac to another Mac and use target disk mode or an external drive to get a known good backup off the drive. This assumes your drive or the Mac is failing and this isn’t just a software error and time you spend looking at the logs or trying to fix it delays the chance to save your data on a good drive. 
Dig a little into the error and try a reinstall of the os. This assumes the data is intact and you’re Ok risking loss and playing the odds. 
Find a technician or spend time learning about the console and boot process and specific boot error (see the logs or boot in verbose mode) to remedy a specific failure in the startup process.