Galaxy S4 Screen Replacement Problem

[JuanTed]

Hi, I've got an issue with my Galaxy S4 that I'm hoping you can help with.

I dropped my phone last week and broke the screen and cracked the digitizer. I bought a new frame / digitizer / screen ready built from eBay, and looked through quite a few online tutorials. Looked straight forward, so I slowly followed the instructions.
The tutorial suggested testing the new screen before completely dismantling my phone, and the new screen works fine and so does the phone. I then swapped over all the components as per the instructions, and turned the phone on. It comes on (Led flashes and it makes a sound when the vol buttons are pushed) but the screen doesn't power up.
I'd assumed that id damaged the new screen / frame, so I put everything back onto my old screen and tested the new one and it worked !!!!

The phone does turn on and my computer recognises it through the usb cable, but the screen doesn't come on.

I may have missed out some important info that you'll need to know, but I'm not very knowledgeable about phones lol

Has anyone ever seen this before, and have any suggestions ?

Thanks

J