If the Power +up/down didn't work after being down correctly a full reset is the final thing to do but it will wipe all data that is on the phone. My advise is try the power +up/down again in case 1st (did you feel and vibration when doing it?) and try also holding down the power button on it's own for 30sec and re try short of that full reset is best bet. 😞 let us know how it goes 
