Try to boot into single-user mode from the GRUB bootloader :
See the following URL for instructions :
From single-user mode, use the command : "redhat-config-xfree86 -v --reconfig" to reconfigure the X Window System (the graphical system of Linux).
Try a lower resolution this time. Start out with "640x480x8bit" to try if that results in a working graphical display.
After you made your configuration changes, type "init 5" to boot Red Hat Linux in graphical mode.
If you get a reasonable picture, press <CTRL-ALT-F1> and login as root.
"redhat-config-xfree86 -v" and this time selected a somewhat higher resolution.
Try to go to graphical mode again with "init 5".
Repeat these steps until you're happy with the way it looks.