Re-install bootstrap code (GRUB)
Please perform the following steps:
Boot from CentOS installation disc (for example, CD #1 or DVD).
For CentOS-4 or CentOS-5 type "linux rescue" at the "boot:" prompt. For CentOS-6 pick "Rescue installed system" from the boot menu.
Mount all filesystems in read-write mode.
Change root to real root ('/') on your hard …