iPhone 4 Screen lighting up but black after flex replacement?

Hi guys,

My friends iPhone 4's power button wasn't clicking and therefore not turning on. So I replaced the power flex cable and prox. sensor, voila the button works and the phone turns on..the screen is lit up but remains black. I looked at various places online and a lot of sites say it's because the LCD's been damaged but I don't think that's the problem as I checked all the ribbons and there seem to be no tears and I was very careful with it.

I did notice one thing when replacing the cable and prox sensor - there was no foam UV filter on the old one or a spacer? The phone seems to have been prevously 'repaired' by someone else as there are loads of screws missing. Would the lack of these two pieces cause the screen to come on but remain 'lit up' black? I've done a few screen replacements successfully on the 4s and 4 but think I may have bitten off more than I can chew on this one.

Any help is greatly appreciated!

Thanks