Umm, have you tried another monitor?
If you have then next best thing is to use the console to edit /etc/X11/XF86config.
Make a backup first
cp /etc/X11/XF86Config /etc/X11/XF86Config-backup_before_vesa
look for this bit (or similar) in the code:
VendorName "Videocard vendor"
BoardName "ATI Radeon Mobility 7500"
Change the Driver to "vesa".
Write Quit and restart X.
If that dont work then look in that /etc/X11/ directory for backups that redhat-config-XFree86 makes. Perhaps someone was messing with the resolution and pushed it too far. Look for a file date around the time when thing used to work.
ls -l /etc/X11/XF86*
Then maybe theres one from the correct time. Replace the dodgy current one with that:
cp XF86Config_backup_name XF86Config.
CTRL-ALT-BACKSPACE or startx to test.