1. Try starting up in Safe Mode, as per > Use safe mode to isolate issues with your Mac - Apple Support
2. Try starting up in macOS Recovery, as per > About macOS Recovery - Apple Support and see if you can Reinstall macOS or Repair your Macintosh HD using Disk Utility.
If neither of those work, then you probably have a Hardware problem and need contact your local Apple Store or AASP to schedule a Service appointment. see > Contact - Official Apple Support