IPhone 4S suddenly won't turn on?

I was walking to class, and I used my iPhone to check notifications. It worked perfectly fine. But as I reach my class, I take it out again, and it won't turn on anymore. The screen stays black no matter what I do. I've tried holding down the power button, but the "out of battery" icon doesn't even appear. I don't understand what happened, I definitely didn't drop it, and there's no apparent physical damage.

Iphone has a powerfull thing don't know what to call it but bayscially
If youre phone does not turn on like that hold doen the top button(the power button) and the other button on next to the screen at the bottom similtaneously for quite some time until it turns on, hold both down together until youre phone turns on and it will turn on eventually
Do this everytim this happens or everytime your phone freezes

Try a reset, hold middle button and power button till the apple logo shows up then let go