Unresponsive iPhone 4s after screen and battery replacement

I have a 4s that would not power up and iTunes would not pick up. I replaced the battery. Screen still showed no signs of life but when plugged into computer iTunes would pick it up. I replaced the screen assembly and now it's back to square one, screen has no display, phone does not charge and iTunes does not recognize when plugged into computer. Any ideas? All connections are secure.