I had this problem several months ago and this solved the problem. It happened again today, but now it will not respond. The only thing that happens is the volume is turned down. :( I guess this only works once.
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== Update ==
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Update: I plugged in my phone to a computer via the USB cable. (I also have the settings set to lock my screen only to open with a 4-digit password). When I plugged the phone into my computer, my screen came up. I put the phone on silent. Only then was I able to press the power button then the volume down button and hold them both until the phone reset. The button works now.
I had this problem several months ago and this solved the problem. It happened again today, but now it will not respond. The only thing that happens is the volume is turned down. :( I guess this only works once.