First try and boot from MacOS recovery (not internet recovery)
- Restart your Mac.
- Immediately press and hold one of the following key combinations until you see the startup screen:
- Command-R: Start up from the built-in macOS Recovery System. Use this key combination to reinstall the latest macOS that was installed on your system, or to use the other apps in macOS Recovery.
If you cannot install macOS this way, you will need to have access to another mac and download macOS High Sierra here: How to download macOS – Apple Support (UK)
After you’ve downloaded high sierra on another mac you can follow these instructions to make a bootalable usb device
Create a bootable installer for macOS - Apple Support
After you’ve created the bootable installer, please insert it into your broken mac and press option on startup. Select the bootable installer as boot device, so you can install high sierra anew