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No power

Try a different Lightening cable. Try lightening to plug rather than lightening to USB or vice versa. If you still have nothing your phone needs servicing as either the battery is no good (probably the case if you were seeing it run out after a couple of hours before the failure) or the lightening port has failed.
Of course, it could be worse and it's bricked for some reason, but those are the usual suspects for no switch on.

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Before doing any of the suggested simply try holding down the power button and the home button for 8 seconds. This will restart the device.

If that doesn't work then try charging it as suggested, using a different cable. Have you had any charging problems previously?

If that fails trying placing the phone into its recovery mode, we will need iTunes on a computer for this. Plug it into the computer and hold both the power button and home button for 8 seconds, after this let go of the power button but keep holding the home button. We should get a message on iTunes.

If this fails then the phone will need a service.