Next step would be to try booting to your recovery partition. Restart, while holding Command + r
When you get to the menu screen, open the Disk Utility, and select your hard drive, then try First Aid.
If that reports no problem, try reinstalling your macOS system. That's one of the choices on that menu screen.
If that does not help, I suspect that your hard drive is probably failing, and you will probably need to replace your hard drive.