Computer shut down installing 8.1 *no boot disk*

Hi, I was using my son's laptop (hp 255) as mine is in for repair and thought I would (helpfully) install windows 8.1 onto his system as a free upgrade (bought in march this year running windows 8) Unfortunately the power failed whilst the update was installing and now I can't boot up the computer and he did not create a system repair disk
I have looked at safe mode (turn on and press esc) and that will bring me to various options but I don't know if any will help - I have tried restore and it appears there are no restore dates available.... Any help (or if this problem has already been posted, direction) very gratefully received!!

Try to do a repair using a borrowed Windows 8 disk. Failing that you could also do a Factory reset using the recovery partition and start all over.

Thanks for the reply, can you explain how to do a factory reset?
These options come up in the boot manager screen but I don't know instructions on how to proceed with rebooting from EFI file.... anyone??
OS boot Manager
Boot From EFI File

Visit your computer manufacturers website on how to perform this action. If unable you could also order Recovery Disks from them if available.

Many thanks, I will check that out with the manufacturer!

my suggestion,is ,before you do a factory restore you should check with your son to make sure he has any important data[pictures ,music ect ect ] backed up somewhere,as a factory restore will wipe his harddrive clean and be like the day it was bought

Will do, thanks!