Yes, you can make a boot disk after the fact, but you need to get access to your root partition or get it to boot up into your linux installation. What version of red hat do you have?
This method will allow you to get access to your root partition. You can usually boot up into rescue mode from the cd install disk or the floppy install disk and then you can mount your root partition and fix the problem. This page here
describes how to enter rescue mode for red hat 7.0. After you get your root partition mounted, then you can create the boot disk.
I have another idea. I have never tried this so it may or may not work. If you get a lilo prompt when you boot up, you could enter in the line ' root=/dev/hdXY '. Replace ' X ' with your hard drive letter. If your drive is the primary master, then it would be ' hdaY '. If primary slave, then ' hdbY '. Then try different numbers for ' Y ' until it boots up. Example: hda1, hda2, hda3, ... You'll have to reboot after each try.
With luck, you'll get in and then you can make a boot disk.
With either of these methods, you'll want to make sure that your root partition is properly entered in your lilo.conf file before you create a new boot disk.