My iPhone 3g suddenly shut off and won't turn on. i did hold the sleep and home buttons simultaneously for 10secs, released sleep and continued holding home for another 10secs then i connected it to itunes. the phone still won't turn on but i heard that sound when u plug it in a usb. the itunes recognized it. i was able to do a back up and restored it to 4.2.1 all through itunes. after finishing everything, the phone still won't wake up / turn on. all it does is make that sound when i connect it to itunes/usb. nothing on the screen, even a black or white light. nothing at all. what should i do next? help please..