Why can't I turn on my smartphone?

Hi,

I was normally playing games on my smartphone. Just like other days. Than, I turned my phone off (because I went sleep). When I woke up, I tried to turn my phone on, but nothing happened. I hold the button for more than 10 seconds, and still nothing. I tried to push volume down+power on, and nothing happened. What's wrong?

Thank you so much for quick response.

Matej Palo