This problem usually after an improper shutdown corrupts your Xdefault file.
Check your /etc/inittab file. In it you will find a line which will say
# Default runlevel. The runlevels used by RHS are:
# 0 - halt (Do NOT set initdefault to this)
# 1 - Single user mode
# 2 - Multiuser, without NFS (The same as 3, if you do not have networking)
# 3 - Full multiuser mode
# 4 - unused
# 5 - X11
# 6 - reboot (Do NOT set initdefault to this)
Set your default runlevel to 3. This will allow you to boot into a command line mode. After that run the "setup" program that comes with Red Hat and use it to reconfigure your XFree86. If you do not have the tool to reconfigure X, you may have to edit the "/etc/X11/XF86Config" by hand. Do not worry, it is well commented. But make a backup copy of the file first. Also check the /var/log/XFree86.0.log before making changes. It may provide some hint on where there is some error.
After you have made the changes, type "startx" and that should get you into GUI mode. If it does not, you may find some more error messages o your scren which will prove helpful