You boot up a Linux Live CD, make a directory on /mnt, mount the unbootable Linux partition on it, change root to the unbootable Linux, do a Grub-install, exit and reboot.
As example if the root partition of your Linux is hda6, after boot up a Live CD (almost any one will do)
Code:
mkdir /mnt/temp
mount /dev/hda6 /mnt/temp
chroot /mnt/temp
grub-install /dev/hda
exit
reboot
If you use a Sata the hda6 should be sda6 and hda should be sda.
hda without the partition number means the whole disk and that is the MBR.
