Well, if it is not starting up then you might use a rescue disk that might have come with your RedHat. If you don't have a rescue disk you can download and burn a live-cd linux like Knoppix
. Knoppix runs entirely off a cd so it won't overwrite anything on the hard drive. Once you are ready, you can boot the knoppix cd (remember to check "boot from cd" is the first boot option in your bios) and open up any text editor and edit the fstab file. Save changes, reboot...and remove the cd.
In any case, you should at least have one live cd handy...for any such future problems.