Support with Reinstalling Mac OS

Try booting into Internet Recovery Mode using Command + Option + R. You can try using Disk Utility First Aid to repair the drive, then try to reinstall macOS over top of itself. You may possibly need to perform a clean install by first erasing the drive before reinstalling macOS (which destroys all data on the drive).

Try running the Apple Diagnostics to see if any hardware issues are detected such as a failing hard drive. However, the diagnostics don't detect all types of drive failures.

If you have the non-Retina model, then maybe the hard drive SATA Cable is bad which is extremely common on the 13" model. You can test this theory by removing the hard drive and attempting to boot it externally using a USB to SATA Adapter, drive dock, or enclosure. Personally I suspect the hard drive has failed after 8 years of use. I hope you have good backups.

The good news it is easy to replace the hard drive with an SSD which will also provide a nice performance boost to this laptop. You may want to replace the hard drive SATA Cable as well since it has an extremely high rate of failure especially when using an SSD. OWC sells this cable.