Install/rescue/restore grub2

To restore grub2 run this commands:

sudo mount /dev/sda1 /mnt

where /dev/sda1 - partition for grub (with installed linux)

sudo mount --bind /dev /mnt/dev
sudo mount --bind /proc /mnt/proc
sudo chroot /mnt
sudo grub-install /dev/sda

If output has "chroot: cannot run command `/bin/bash': Exec format error" then your live CD architecture doesn't match with restorable.