# Mount root partition:
sudo mount /dev/sdXY /mnt # /dev/sdXY is your root partition, e.g. /dev/sda1

# If you have a separate boot partition you'll need to mount it also:
sudo mount /dev/sdYY /mnt/boot

# Mount your virtual filesystems:
for i in /dev /dev/pts /proc /sys /run; do sudo mount -B $i /mnt$i; done

# Chroot
sudo chroot /mnt

To add boot-repair to the repository

sudo add-apt-repository ppa:yannubuntu/boot-repair

To Update your repository

sudo apt-get update

To install boot-repair

sudo apt-get install -y boot-repair

Once Installation complete run boot-repair on terminal by typing the following command or select it by System->Aministration->Boot Repair.

boot-repair

NOTE: Update the Boot Repair if its newer version is available.