Postagem original de: Dan ,
If the GPU/video controller was defective you would have no external video so I doubt replacing the video controller would fix things here. You likely have either a CFL issue or the inverter has gone. Whats odd here is Profile doesn't show your display. Are you sure you were looking under the right heading '''Graphics/Displays''' Click on the heading and it should show info for both displays iMac and External. Have you tried running [http://support.apple.com/kb/HT1509|Apple Hardware Test]? Looking at your display at an angle you should see a faint image of your desktop (with laptops you can often use a flashlight to shine through the Apple logo here you can't). If you can then you have video only the lighting of the display has a problem.