Boot from any live CD or something like SystemRescueCD, basically anything that runs Linux with GRUB on it:
1) Become root (if you are not already)
2) "grub" will get you the grub prompt
3) "find /boot/grub/stage1" will print out where your install is located, should be something like "(hd0,0)"
4) "root (hd0,0)" or whatever from the previous step.
5) "setup (hd0)" to setup grub in the MBR on that drive.