If you have repaired your system from a backup tape (by dump or tar), for example, you will need to repair the bootstrap area.You need to re-install the bootstrap code that is grub.
Please perform the following steps:
Boot from OS(Redhat,Fedora,CentOS) installation disc (for example, CD #1 or DVD).
Type “linux rescue” at the “boot:” prompt.
Mount all filesystems in read-write mode.
Change root to real root (‘/’) on your hard disk:
# chroot /mnt/sysimage
Re-install bootstrap code (GRUB).
If you wish to re-install GRUB to the MBR …
