So I had similar problem and I was so frustrated with it I wanted to try to help others that were experiencing it. So I bought a screen replacement and I finally figured out that it was the camera on the new screen that was causing it. I have read on other forums that some had to replace the proximity sensor too. I would suggest open up iphone and unhook the camera from the motherboard and see if your iphone boots. If it does then it is your camera. You can either keep it unattached and reclose if you don't care not having a front facing camera (i would almost cut the ribbon tbh or take the camera out and just not have it at all if determine camera is causing it) or install the camera from your old screen (this is what i did once I figured out it was front facing camera. Best of luck!