A month ago my dad told me that his GNU/Linux Ubuntu Hardy screen resolution stuck at 640x480 because somebody had fiddled with his computer to fix something else. So, I googled around and found some proposed methods none of which worked. At the end, I reinstalled the whole system to fix the screen resolution. But, I will list the proposed methods I found for anyone else and my reference. I will improve it if I find new proposed methods. And, you are very welcome to improve it.
1. The dead `displayconfig-gtk'
2. The no longer available: `nautilus /usr/share/applications/' and go to `Screen and Graphics'
3. dpkg-reconfigure xserver-xorg
4. Delete all failsafe X org configuration files in /etc/X11
5. Be careful in changing your monitor since the auto-detection of the new monitor may fail and the resolution will break when using the old monitor again (those who service other people's PCs should be careful here since the customers usually don't bring their monitors with them when they bring their PCs in your shop)
6. There are reports that upgrading to Jaunty from Intrepid break their Intel VGA performance: https://wiki.ubuntu.com/ReinhardTartler/X/RevertingIntelDriverTo2.4 and http://ubuntuforums.org/showthread.php?t=1130582.