Usually the last thing done by Linux installers is the boot loader.
If everything else is installed properly, you can boot off a LiveCD or Redhat CD with rescue option (assuming it has one - I'm not a Redhat user), mount the partition and chroot into the system and install GRUB.
eg
Code:
mkdir /mnt/tmp
mount /dev/hdaX /mnt/tmp
chroot /mnt/tmp /bin/bash
This should have you running in the installed Redhat OS, then
Once in grub, (assumming grub.conf installed by installer
Code:
root (hd0,0)
setup(hd0)
quit
Reboot and hopefully should be OK.
** Note /dev/hdaX is replaced by you HD location and (hd0,0) also changed to reflect your HD location)