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.

ls
# Select correct drive. If you don't know just try all
set prefix=(hd1,gpt5)/boot/grub
insmod normal
normal

# It's booting now...

sudo update-grub
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 update-grub
sudo grub-install -v --target=x86_64-efi --recheck /dev/sdb