How to repair broken MBR in RHEL 6 ?

1) Go to the grub command line when the system boots up.
2) Find the partition which contains the grub.conf file.

grub> root
  (hd0,0): Filesystem type is unknown, partition type 0x8e

3) Reinstall grub on the returned partition.

grub> setup (hd0)

4) The above command should fix your broken MBR, now reboot the system.

grub> reboot