Edit the driver line for your video card in /etc/X11/XF86Config (or XF86Config4 on some distros). You can change it to 'vesa' and that will almost surely get X up and running again. You'll still want to look into trying to get the radeon driver working again, but at least you'll be able to research within a functional X session.
The video card section will look similar to this:
Just change the driver line to vesa and then try to start X again.