Why has my iPhone 6s suddenly stopped working?

My Iphone 6s has suddenly stopped working for no real reason. I just could not turn it on one day (It´s charged and I have tried hard resetting it). I haven´t dropped it or anything like that and there is nothing wrong with it´s exterior except for one of the two little screws on the bottom has disappeared. I would really like to know what´s wrong whit it or what might have happened. Thx in advanced :)