So i got an iphone 4 cdma in a trade and it had a cracked screen at the time, after two improperly fitting screens i figured out i had the cdma model, but what im having problems with now is that the LCD screen will light up, and i can toggle the vibrater on and off, but i cannot get the screen to truly show anything, i can plug it into my computer and it shows it works, the memory... etc, any suggestions on what i can look at to fix this, screw placement is my main idea for what's causing this