I just tested upgrading a Red Hat Linux 9 machine to CentOS 5. The upgrading process ran smoothly except after upgrade the X server did run.
Usually, the problem might be due to the installer uses horizontal or vertical frequency that the monitor could not handle. The fix of this kind of problem is easy, I only need to modify the /etc/X11/xorg.conf file and restart X server. However, it didn’t work this time.
I then tried to start system-config-display , but the monitor complained about the signal is not displayable. I was not able to use system-config-display at all.
I used X forwarding from a remote machine and I hoped that I can use system-config-display this way. The system-config-display ran fine, but when I restarted X server, the server still could not start.
Finally, I found that the driver was not installed through the log file. After I did a yum install xorg-x11-drv-ati , the X server was able to start.