Okay so my iPhone 3G won't turn on. It was working perfectly fine until it just shut off when I was using it. Now it won't turn on. If i press the lock button to power it back up, it powers up as usual, showing the apple logo. Then, a few seconds later, the apple logo disappears and the screen goes black and it shuts off again. I tried charging it but it didn't help. It was on full battery before it shut off on me. I don't know what to do. I can't use my phone and I need it!