iPhone screen is white after replace

Hello! I broke my iPhone 3G’s LCD so I bought a new one. I replaced it by myself and everything seemed fine but when I turned on the iPhone the screen was all white. I searched for others posts but I don’t think it’s a software-related issue because the iPhone syncs with the PC (so I can exclude DFU mode). What can I do? The three flex cables seems connected well. Do you think that my new screen is already broken?
Thanks in advance!