First off where did you buy the replacement screen? Second did that screen also come with new ribbon cables?
If you bought the replacement online and just installed it without reading the instructions or it didn't come with any then you have the reason there.
If you did read the instructions and followed the steps correctly, then did the replacement come with new ribbon cables and connect lines is the next part, because the reason the first screen failed was not only the screens dynamics, but the constant open and closing, bending causes the cables to get worn in the same area and eventually it will tear or just break. And since you stated that the hinges had broke first that automatically says that the screen will fail due to not being able to function as it should. So with that said, and your answer was YES to the new cables, then final step is to retrace your steps when installing. Because you forgot to reconnect something to have the screen light up. If the white light on the screen doesn't light up you will see what you see now.
I suggest you to goto ifixit.com
they have a step by step with images to install or replace parts on all Apple portables. Just follow their instructions correctly to find your PowerBook and then use their install steps.