You can re-install grub boot loader in rescue mode by issuing the following command :

#grub-install /dev/sda1 or hda1 ( based on your boot partition)

if it is scsi drive it would be sda and hda if it is ide drive.

If you get something like this :

” Error /dev/hdx or sdx does not have any corresponding bios drive ”  then issue the following command :

#grub-install –recheck /dev/sda or hda

Hope it helps