Restoring GRUB2 on Ubuntu

https://help.ubuntu.com/community/Grub2#Reinstalling%20from%20LiveCD

METHOD 3 – CHROOT

This method of installation uses the chroot command to gain access to the broken system’s files. Once the chroot command is issued, the LiveCD treats the broken system’s / as its own. Commands run in a chroot environment will affect the broken systems filesystems and not those of the LiveCD.