I would check to see if the pot that controls the contrast is functional with a multimeter. If it isn't, replace it and see if that solves the issue. I would also check all components on the PCBs for signs of damage, e.g. burnt transistors, charred board, leaky/dry electrolytic caps... If you identify any bad components, solder in a replacement and see if that works. There are some decent Game Boy schematics on dev'rs if you need. Link: http://www.devrs.com/gb/hardware.php#har... From there, there's not a lot of places to go. The most likely cause of the issue is what Andrew said. The thin ribbon cables used to drive the LCD were actually sandwiched inside the panes of glass that constitute the LCD. There is a thin rubber strip with adhesive running along the bottom edge of the LCD to create pressure when the case is closed in order to maintain a solid connection. Over time people would frequently (even if inadvertantly) expose their Game Boys to sun or other heat sources and these ribbon cable...