I have the ATI graphics manager (Catalyst Control Center) configured for my external monitor, attached to my Toshiba laptop, and the external monitor is recognized by this utility. However, the monitor shows up as "unknown" in the Ubuntu> monitor> preferences, and the monitor itself shows the message "No Signal". The laptop is dual boot, with Windows Vista, and I have the same problem with the monitor signal if I try it in Vista. However it was all working fine only days ago in Vista.
The same external monitor works fine on an ancient Dell, and is recognized in monitor> preferences, so I know it is not a problem with the monitor or the cables.
I see that there is a file xorg.conf (/etc/x11/) that seems to hold all the right info regarding the ATI graphics card, and another xorg.conf (in /usr/share/doc/xserver-xorg-video-nouveau/examples), which has minimal info. Do I need to be editing these?