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