Why is the screen not coming on after touch digitzer install?

[AC Question]

I cracked the screen on my Samsung Mega so bad that I could not even touch the screen to do anything. I replaced the screen and the digitzer. I know the phone turns on because it lights up and plays the start music. Sometimes the display will show and sometimes it will not show. For example, I put the battery in and turned the phone on without the back part of the case. The phone worked I was able to text and make a phone call. I put the screws back in and the case on the phone and the screen started to black out. I am assuming it has something to do with the pressure being applied to the phone. How do I fix this?