Why is there no display on screen?

i have fixed iphone 4 before and never come across this problem.

one of my friends broke her screen on her iphone 4 i ordered a several replacements from eBay.

the iphone just shows the back light and no image i have check the cables connecting to the board several times and still nothing the phone boots up because it vibrates at the beginning so not sure what issue is

has anyone else had this same problem???