replaced rear housing, now phone won't start or appear in iTunes

The rear housing of my iphone 3g was cracked, so I replaced it. Now iTunes won't recognize the phone, nor can I get it to boot up. When I hold down the power button for a few seconds, the black screen shows some even darker black lines at the top. Those lines eventually go away. I've taken it apart and reassembled many times, but no luck. The battery is still good (3.7v per the multimeter).