Try starting in Recovery mode (hold down command & r when turning on the mac) and see if the Drive (HDD or SSD) is listed there. If so you can try running First Aide (icon at top of window) on it and see if it can fix an file system related issues.
If the drive passed and it still won't startup then you probably have a software issue preventing startup. Hopefully you have a recent backup you can restore otherwise the Recovery mode also has a way to reinstall the OS.
This basically means your hard drive, or the connection to it, is damaged or corrupted. If you are lucky, as the other poster has said, you'll be able to restore from a backup, but the alternative is getting a new hard drive cable, or a new hard drive/ssd altogether!