Phone not responding

Make sure you follow these steps exactly. Press and very quickly release the volume up button, press and very quickly release the volume down button, then press and hold the side button for about twenty seconds. When the Apple logo appears, release the button. It may take a few tries.

If it really doesn't work, it's a hardware issue. You'll have to take it in for service - Find Locations - Apple

I sounds like you need to get an appointment at an Apple Store or Apple authorized service center for help with this. See if this helps: If your iPhone won't turn on or is frozen – Apple Support (UK)