This could be an issue with the display data cable.
iFixit Display Data Cable.
I used to fix Nextel cell phones and a BIG issue was that since the display cable was a ribbon cable it would wear out and break over time. This was because of the open and closing of the device. Friction over time broke the cable.
This is possibly the reason why your display is only lighting up and not showing a picture at all.
Other possiblities would be the Display Inverter, but it doesn't seem likely. Another could be the Logic Board or the Graphics Card.
I would try replacing the Data Cable first. you would be able to tell from looking at it if it is the problem. There would be a crack or tear close to the hinge area. Heres a break down of the display for the PowerBook G4.