Use a live cd and check the contents of /boot/grub/menu.lst
Make sure that the relevant versions of initram and vmlinuz are in /boot/
If you can't see anything wrong, you can just reinstall grub:
1. Boot off the 1st RedHat CD
2. At the boot prompt type 'linux rescue'
3. The rescue mode will find the RedHat installation
4. Type 'chroot /mnt/sysimage' (without quotes)
5. Type 'grub-install /dev/sda' (to install Grub to the MBR of the 1st HD) - it might not be 'sda' on your system.
6. Restart and you should get the boot menu
Alternatively, you could use the supergrub