phone will turn on but the screen stays black

Hi

I replaced a broken screen on my grand son's phone. The phone will not make the sound like it is coming on but the screen stays black. What is causing this, is it fixable or should I look for a LCD screen for it. I did noticed that the little wire that connects both of the motherboards has a few freys in the wire. Is this bad do I need to buy a replacement of that too. I have taken the phone completely down to check to make sure everything was connected and there is nothing lose.

Thank you