Hi, answering your question can be a bit difficult because there could be a multitude of things wrong or just something simple. First off the likelihood of it being a chip is very small and could really only be caused by two things: someone damaging the phone by taking it apart and water damage. If it "suddenly" went black from normal use then its probably just the connector is not fitted properly, which your friend or you should have noticed while taking it apart. So if it's properly connected and is still not working then it could be the lcd or backlight; my recommendation take it to an iPhone repair shop with reasonable quality service and ask them if they'll test a replacement screen on the phone. They probably will because if it turns out to be the screen they can sell you their services.