Please perform the following steps:
Boot from 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 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
Alternative Recovery from GRUB Issues – Super Grub Disk
Super Grub Disk has a variety of open source boot recovery tools. For CentOS legacy GRUB SuperGRUB works well.