See if you can boot into Safe Mode.
Try booting into Recovery Mode (Command + R) and Internet Recovery Mode (Command + Option + R) and run Disk Utility First Aid. Make sure to examine the "Details" to see if there are any unfixed errors listed (even if First Aid says everything is Ok. If there are unfixed errors, then you will need to erase the drive and reinstall macos or restore from a backup. If after running First Aid the Mac won't boot normally, then boot into Recovery Mode and select "Reinstall macOS" to install macOS over top of itself. This should not affect your data, but hopefully you have a backup just in case.
What is the exact model of your Mac? You can get this information by entering your serial number here:
https://checkcoverage.apple.com/
What version of macOS are you using on this Mac?
Try running the Apple Diagnostics.