Hi, i've taken apart my iPhone 3g several times now (got wifi problems) and the first time I put it all back together my screen wasn't working as well. I re-opened it, and noticed that the very last screen connector (the one with the cornered shape) comes loose very easily. Check that connector again, and push it in until you feel or hear a click. Do not lift the screen up again, as it will then bounce up again. It worked for me. Also, before closing the whole iPhone again, turn it on first to see if it has worked, and if not keep trying.
Hope it works for you.
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