Sounds to me that the contact points in the front ribbon cable (below screen) have come undone, or loosened over time, a common problem in older DMG's. All you near to do is open the console and remove the rubber gasket from around the screen. The, see the brown ribblo cable directly below the screen, that is what the problem is. You near to turn the contrast all the way up to make the screen black, then take a soldering iron and lightly press on the ribbon right along the bottom of the screen, and run the iron back and forth until the you can see the image (keep power on the whole time) this should work but good luck anyway. Also, do this at your own risk, as I am not responsible for damage:-)