phone turned off and not turning back on
If it's a software problem, you can try holding the power button and volume down button down at the same time and keep holding for at least 10 seconds.

The phone should now boot up and give you a list of options. Use the volume up/down arrows to navigate and power button to select. Find the 'Reboot' option and select it to reboot your phone.

If that doesn't work, could be a hardware problem, which would mean you will need to take it in for repair