# This method requires you to boot from a Linux live USB.
# (sdXY is your boot partition)
fdisk -l
sudo mount /mnt /dev/sdXY
mount -t proc none /mnt/proc
mount -o bind /dev /mnt/dev
mount -t sysfs sys /mnt/sys
sudo chroot /mnt /bin/bash
update-grub
/usr/sbin/grub-install --recheck --no-floppy /dev/sda
sync & reboot