I am assuming you tried a windows repair on it by booting from W10 media first right? Ok if that fails try this:
Depending on how you are accessing it one great way to fix it is to boot from a recovery/dart disk and recreate the partition using bcdboot.exe
bcdboot c:\windows /s h: /f UEFI
Have a look at the command, I have used this to fix an unbootable disk first with bcdedit then with the bcdboot but you will need to investigate the current state of your disk to determine which you will use or the order which you will need to repair the boot partition.
I have had limit success with the bootrec command and UEFI/GPT disks.
