If you have repaired your system from a backup, or have installed another OS that may have overwritten the master boot record (MBR), you will need to repair the bootstrap area.

Re-install bootstrap code (GRUB)

Please perform the following steps:

1.Boot from CentOS installation disc (for example, CD #1 or DVD).

2.For CentOS-4 or CentOS-5 type “linux rescue” at the “boot:” prompt. For CentOS-6 pick “Rescue installed system” from the boot menu.

3.Mount all filesystems in read-write mode.

4.Change root to real root (‘/’) on your hard disk:

# chroot /mnt/sysimage

5.Re-install bootstrap code (GRUB).

##If you wish to re-install GRUB to the MBR on a SCSI or SATA disk (/dev/sda):

# grub-install /dev/sda

##If you wish to re-install GRUB to the PBR of Partition #2 on a SCSI or SATA disk (/dev/sda2):

# grub-install /dev/sda2

##If you wish to re-install GRUB to the MBR on an IDE disk (/dev/hda):

# grub-install /dev/hda

##If you wish to re-install GRUB to the MBR on a HP Smart Array disk (/dev/c0d0):

# grub-install /dev/cciss/c0d0