welcome to LQ!!
First, what version of Redhat?
Are you getting a graphical login screen or just a terminal display?
When you first boot up, try entering ctrl-alt-F1 (or maybe F2 or F3) to get a terminal. Then, log in as root. If that is successful, then run
grep -B5 "(EE)" /var/log/Xorg.0.log
and look for evidence that it cannot find the right video driver. In this case, tell us what your video card is.