Boot from a Live CD :
Install the GRUB2 boot loader:
sudo mount /dev/sda5 /mnt
sudo grub-install –root-directory=/mnt /dev/sda
That’s /dev/sda — the hard disk itself, not the Ubuntu partition – /dev/sda5.
Unmount the Ubuntu partition and restart the computer like so:
sudo umount /dev/sda5 ; sudo reboot
If you have more than one OS installed, re-detect OSes like so:
sudo update-grub