During Windows 10 update some restarts are necessary. Today first update restart broke my Grub and computer refused to boot. I went to BIOS and changed bootloader from Grub to Windows loader. Then Windows update proceed correctly.
Then I swap boot loaders again and saw grub prompt.
# Select correct drive. If you don't know just try all
# It's booting now...
sudo grub-install /dev/sdb
# In case of Could not prepare Boot variable: No space left on device:
sudo rm /sys/firmware/efi/efivars/dump-*
sudo grub-install -v --target=x86_64-efi --recheck /dev/sdb