Why is my iPhone 4s screen black?

I dropped my iPhone down the stairs and it cracked into a million pieces. So I ordered a new front and back and changed it myself. When I tried to turn it back on the screen was completely black. I can hear noises (ie: people ringing me, lock noises, texts) but the screen will not come on. I followed all the steps and put everything back how it was. I've tried holding the lock and home buttons down together but that didn't work and I've also rebooted the phone on itunes and that still doesn't seem to be working!:(

any ideas?!?!?!