A. While not exactly a Windows topic, many IT pros have iPhones and have asked this question. Fortunately, it's easy. Just hold down the Power button and the Home button (the circle at the bottom of the phone) for 10 seconds. The phone will reset and should be working again. If this doesn't work, it may be a serious problem, so you should go to an Apple store for help.
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Check out hundreds more useful Q&As like this in John Savill's FAQ for Windows. Also, watch instructional videos made by John at ITTV.net.