Why my phone won't turn on?

What kind of charger are you using; an original Apple charger or a dollar store special? Has anything else happened to your phone in the last few days, like a big drop or impact, contact with water or a repair?

If it just died out of the blue, then there could be a few possibilities. A bad battery for example (if you noticed reduced battery performance) or it could be logic board damage. This is usually caused by cheap chargers but again, depending on the circumstances, it could be almost anything.

Try connecting the phone to an iTunes enabled computer. If the computer recognizes the phone, that means it is booting. Shine a bright light on the screen; if you can see a dim image, then it would be a backlight problem.

Let us know what you find by commenting on my answer.