It doesn't sound like it's a problem with the buttons, it sounds like you disconnected the screen during your original install.
You already took it apart once, sit down with a guide and do it again only this time pay attention to the screen connectors. Unplug, verify direction, re-plug. Try it again.
Failing that, put the buttons back in and RMA it.
Or, buy a replacement and sell that one cheap as a "fixer upper."