grub mbr reinstall

sudo mount /dev/sdXY /mnt 
#Example: sudo mount /dev/sda5 /mnt
sudo grub-install --boot-directory=/mnt/boot /dev/sdX 
#Example: sudo grub-install --boot-directory=/mnt/boot /dev/sda
Reference: https://help.ubuntu.com/community/Grub2/Installing#via_the_LiveCD_terminal