Why do I have a white screen after replacing lcd and digitizer?

I bought a replacement screen for my ipod touch because my screen was cracked and the right side of the screen was unresponsive. i've fixed several ipod's before and this is the only one to have this problem with. I'm not sure if it's a faulty lcd and digitizer because i've used this seller before but it might. I have triple checked the cables and i'm not sure why i'm having this problem. The ipod won't power on by me pushing the power button or holding the home and power button. it's not in dfu mode. the only way i can see the white screen is if i plug it into the computer and itunes recognize it. Then the white screen shows for a couple seconds then flicks off. I'm not sure if i'm missing something. i have tried to ground it but not to the computer to a wall plug in for the ipod. I'm up for any suggestions and i'm really close to buying another replacement screen but only after i've exhausted every option. Thanks for your patience.