The reason for your booting problem is that your boot loader might have been corrupted. In many cases, the GRUB boot loader can mistakenly be deleted, corrupted, or replaced by other operating systems.
The following steps detail the process on how GRUB is reinstalled on the master boot record:
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.conf file, as additional entries may be needed for GRUB to control additional operating systems.
6. Reboot the system.