Try the steps here:
How to reinstall macOS from macOS Recovery - Apple Support
1. Start up from macOS Recovery
To start up from macOS Recovery, turn on your Mac and immediately press and hold one of the following sets of keys on your keyboard. Release the keys when you see an Apple logo, spinning globe or other startup screen.
Command (⌘)-R
Reinstall the latest macOS that was installed on your Mac (recommended).
Option-⌘-R
Upgrade to the latest macOS that is compatible with your Mac.
Shift-Option-⌘-R
Reinstall the macOS that came with your Mac, or the closest version that's still available.
If that does not work. you can try and make a bootable installer from another mac:
How to create a bootable installer for macOS - Apple Support
Good luck!