Why my device does not turn on?

No Power issues can be complex in any device due to the number of factors, but here's a couple things I would do to double check what might be going on.

1. Try a different charger (a known good one if possible) or verify your charger on another iPhone.
2. See if charging wirelessly makes a difference. Even if you don't have a wireless charger, they are relatively inexpensive these days. Plus if your phone comes on, then you'll know it's something in the USB chain that's at fault.
3. Your battery could be the issue. If it was already failing or on the verge of, and it went flat dead, it's possible it just doesn't have enough oomph left to get the phone to boot.

Otherwise this could be lots of things.