IPhone 5s won't turn on?

I was just taking pictures with my iPhone 5s and then I turned it off and now it won't turn back on. I've tried pressing the on of buttons for a while like when it restarts but it won't work.

Added (1). It will not show the dead sign when I plug it in to charge it and it was around 70% when I was taking the pictures.

Is it dead?

You can't just mash away at that button. If you want to turn it on, you have to be gentle, take your time, caress it.