How to Repair a broken Grub

First Boot from a live cd, mount the linux partition, add /proc and /dev and use grub-install  /dev/xyz. Suppose linux lies on /dev/sda6:


# mount /dev/sda6 /mnt   ------ mount the linux partition on /mnt

# mount --bind /proc /mnt/proc ----- mount the proc subsystem into /mnt

# mount --bind /dev /mnt/dev ------- mount the devices into /mnt

# chroot /mnt  ---------- change root to the linux partition

# grub-install /dev/sda -------- reinstall grub with your old settings