I think there is a tip about this in the Linux tips and tricks sticky ...
Anyway, to start with, you need to boot into a linux environment, so that you can reinstall GRUB. Any bootable floppy or CD will do, but if your distributions CD has a rescue mode (generally accessed by typing "linux rescue" at the prompt) its better. Redhat does have one, so that makes the job easier.
Once booted, RH will automatically mount your partition on /mnt/sysimage (you may to specify the partition or distro to rescue), but we need to make that directory as the root or '/' directory, so you can type
Now install grub on the MBR
Now you should have grub back
