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Not a good think to have a power failure during an installation or upgrade—certainly you need to be on the mains, not battery power.
I would attempt to boot into Recovery and reinstall the macOS from there.
You may first Boot into Recovery (Command R) and from the dropdown menu: Utilities> Disk Utility> run the First Aid on your Macintosh HD. If errors are found and repaired, run again until no errors reported.
If all that fails you can try Internet Recovery (option command R) at boot up. If Recovery can not be accomplished it should automatically try itself an Internet Recovery—but it does not always happen.
If you have a bootable installerUSB for your High Sierra you can hold the option key and boot off that for the exact same tools.
How to create a bootable installer for macOS - Apple Support
Not a good think to have a power failure during an installation or upgrade—certainly you need to be on the mains, not battery power.
I would attempt to boot into Recovery and reinstall the macOS from there.
You may first Boot into Recovery (Command R) and from the dropdown menu: Utilities> Disk Utility> run the First Aid on your Macintosh HD. If errors are found and repaired, run again until no errors reported.
If all that fails you can try Internet Recovery (option command R) at boot up. If Recovery can not be accomplished it should automatically try itself an Internet Recovery—but it does not always happen.
If you have a bootable installerUSB for your High Sierra you can hold the option key and boot off that for the exact same tools.
How to create a bootable installer for macOS - Apple Support
Computer keep restarting after update attempt
