Same thing happen to me a couple of time when replacing the screen. I found the issue being that the flex cable isn't making a good contact to the motherboard. I've found that most of the times all you need to is open up the iPhone again and make sure that flex cables are making a good connection and that it click onto the motherboard. If that doesn't fix the problem then you have a faulty screen, but 95% of the time it'll be the flex cable connection.