iPhone 4 has black screen

Alright so i have an iphone 4 and have done a few things to it. red conversion and black mid-fame. after all that it worked perfectly fine except my power/sleep button wasnt working. So i went back in found out that i needed to switch back to the original button. and also needed a new cable.

Now the problem. I put it back together and now my screen is blank (black). but the black light is still working and i can hear calls and texts and all that.

new screen the problem? or main board? also i read here on a 3g that i might of confused the main board?