sudo mount /dev/sda5 /mnt
sudo mount --bind /dev /mnt/dev
sudo mount --bind /sys /mnt/sys
sudo mount --bind /proc /mnt/proc
# sudo mount /dev/sdaX /mnt/boot
sudo chroot /mnt
sudo grub-install --root-directory=/mnt /dev/sda
sudo update-grub
exit
sudo umount /mnt/dev
sudo umount /mnt/sys
sudo umount /mnt/proc
sudo umount /mnt/boot #Only if you mounted it earlier
sudo umount /mnt/