The options are spelled out in the articles baby boomer posted above, but I can re-state them here:
1) First, use Disk Utility to Repair/First Aid your drive.
Since your Mac is not working normally, you can invoke Recovery Mode by holding Command-R at Startup.
Recovery Mode contains several useful functions, including a special version of Disk Utility that does not need the rest of MacOS to be running to do its job of repairing your disk.
"Regular" Recovery gets the Utilities from special partition on your normal boot drive. If you see a spinning globe, you have invoked (or fallen back to) Internet Recovery, and you see the Globe because it is fetching the Utilities from the Internet. This takes a little longer, so be patient.
About macOS Recovery - Apple Support
Once you get Disk Utility running, this article describes how to repair your disk:
Repair a disk using Disk Utility on Mac - Apple Support
Once Disk Utility reports your drive has been successfully repaired, you can try booting MacOS in the regular way. If that works, you have fixed the problems and you are done.
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If Disk Utility made some repairs, but there are still some problems remaining, you should run it again until your drive comes clean, or Disk Utility reports it cannot be repaired.
If disk Utility suggests you should ERASE your drive, that is a radical step -- this will delete all your files with no possible way of recovering them. Post back here for additional options before you do that.
2) Second re-Install-in-place.
Your MacOS is likely damaged. It is very difficult to repair the individual pieces, so it will need to be re-installed. You can only Re-install onto drive that is working correctly. If Disk Utility was not able to fix all the problems on your drive, there is no point in attempting a re-Install. the drive must be fixed first.
By design, a Re-install-in-place does not bulk-erase anything outside of System directories. Your added Applications, Preferences, and User Files are not modified.
You can do this directly from Recovery mode. The software for the Installation will be downloaded from the Internet. This may take an hour or more. Then it will need to be installed, which may take an hour or more.
How to reinstall macOS - Apple Support
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