Hi there, based on your descriptions, I suggest you troubleshoot the drive in macOS Recovery mode and then reinstall macOS. For data security, you'd better back up everything on your Mac beforehand. If you cannot back up your Mac, iBoysoft Data Recovery Software is here to help retrieve all files on it.
With all the important files at hand, now it's time to deal with the trouble in a professional way. Here is how to troubleshoot your hard drive and then reinstall macOS.
Step 1: Restart the computer. Immediately, at or before the chime, hold down the Command and R keys until the Apple logo and progress bar appear. Wait until the Utility Menu appears.
Step 2: Select Disk Utility and click on the Continue button.
Step 3: Select the indented (usually, Macintosh HD) volume entry from the side list.
Step 4: Click on the First Aid button in Disk Utility's toolbar. Wait until the Done button activates, then click on it.
After this, you can reinstall macOS then.
Step 5: Quit Disk Utility and return to the Utility Menu.
Step 6: Select Install macOS and click on the Continue button.
Follow the on-screen wizard to complete the reinstallation process. If this way fails to make your device reboot, read this article for more solutions: MacBook Pro/Air Won't Turn On or Start? Make it Boot Now