Below mentioned are the steps to reinstall a corrupted or mistakenly deleted grub (for RedHat and CentOS systems only) using the rescue mode ::
  1. Boot the system from any boot installation medium like a RedHad #1 CD-ROM or a Flash Drive, etc.
  2. Use the linux rescue command as shown below at the installation prompt to enter the rescue environment:
    linux rescue
  3. Type the command below to mount the root partition:
    chroot /mnt/sysimage
  4. Type the command below to reinstall the GRUB boot loader, where /dev/hda is the boot partition.
    /sbin/grub-install /dev/hda
  5. Go through the /boot/grub/grub.conf file once again, as additional entries may be needed for GRUB so as to make any custom changes there (like controlling another installed operating system)
  6. Finally, reboot the system
NOTE :: Use the above mentioned steps at your own risk.