Iphone 4 turned off and won't come back on?

Okay so i sat my phone down and when i came back it it was off. My lock button does not work so i usually plug my phone into a charger or computer to turn it on. It won't power on, it had 85% battery when it cut off. It is not damaged or anything. Please help

Added (1). Also when i plug it into the computer, it doesn't show up on itunes

Sorry, this isn't a real answer, but my phone literally did the same thing a few days ago. I've tried everything possible, and it still hasn't come on. I'm taking it to Apple today and they should replace it, since I'm still under warranty and both the button stopped working and the phone going dead are not my fault.

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