How to Repair, Restore, or Reinstall Grub 2 with a Ubuntu Live CD or USB

grub.md

    sudo mount /dev/sdXY /mnt

[It is usually a EXT4 Partition. Replace the XY with the drive letter, and partition number, for example]

    sudo mount --bind /dev /mnt/dev &&
    sudo mount --bind /dev/pts /mnt/dev/pts &&
    sudo mount --bind /proc /mnt/proc &&
    sudo mount --bind /sys /mnt/sys

[Now we jump into that using chroot.]

    sudo chroot /mnt

[This time you only need to add the drive letter (usually a) to replace X, 
for example: grub-install /dev/sda, grub-install –recheck /dev/sda.]
grub-install /dev/sdX
grub-install --recheck /dev/sdX
update-grub

[Now grub is back, all that is left is to exit the chrooted system and unmount everything.]

exit &&
sudo umount /mnt/sys &&
sudo umount /mnt/proc &&
sudo umount /mnt/dev/pts &&
sudo umount /mnt/dev &&
sudo umount /mnt