since nobody should have to go 10 minutes thinking that their iPhone has just automatically
turned itself off and not be able to turn it on again for no reason (not wet, not broken, not full -- it's a 64 GB capacity)
i just found a link that showed how to restore the power to the perfectly working iPhone 5s that just went off - out of the blue.
only takes two minutes to restore power:
(took me a half hour to locate this and to try it out)
Link:
https://discussions.apple.com/message/23276942#23276942 Follow the last piece on the bottom - 3rd entry - it's written by "Shaik" - where it says to hold down alll 4 buttons, and then power off and then power on.