The othe way:reinstall GRUB
steps:
1) Boot the system from an installation boot medium.
2) Type linux rescue at the installation boot prompt to enter the rescue environment.
3) Type chroot /mnt/sysimage to mount the root partition.
4) Type /sbin/grub-install /dev/hda to reinstall the GRUB boot loader,where /dev/hda is the boot partition.
5) review the /boot/grub/grub.config file,as additional entries may be needed for GRUB to control additional pperating systems.
Reboot the system.