Disclaimer: This answer borrowed verbatim from StackOverflow:
You will be doing this from Windows 10. No bootable media required.
bootsect /nt60 and the Ubuntu live with the
boot-repair suggestions have failed, this has worked for me:
(This answer borrowed verbatim from here)
cmd.exe process with administrator privileges
list disk then
sel disk X where X is the drive your boot files reside on
list vol to see all partitions (volumes) on the disk (the EFI volume will be formatted in FAT, others will be NTFS)
Select the EFI volume by typing:
sel vol Y where Y is the
SYSTEM volume (this is almost always the EFI partition)
For convenience, assign a drive letter by typing:
assign letter=Z: where Z is a free (unused) drive letter
exit to leave disk part
While still in the
cmd prompt, type:
Z: and hit enter, where Z was the drive letter you just created.
dir to list directories on this mounted EFI partition
If you are in the right place, you should see a directory called
cd EFI and then
dir to list the child directories inside
rmdir /S ubuntu to delete the ubuntu boot directory
Assuming you only ever had two operating systems (Win 10 & Ubuntu) you should now be able to boot directly to Windows without hitting the black grub screen.