What happened to my MacOS?
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.