Did you already try to force restart the phone, as described in this article?
...
If iPhone isn’t responding, and you can’t turn it off and on, try forcing it to restart.
Force restart an iPhone with Face ID, iPhone SE (2nd generation), iPhone 8, or iPhone 8 Plus
- Press and quickly release the volume up button.
- Press and quickly release the volume down button.
- Press and hold the side button.
- When the Apple logo appears, release the button.
Note: If iPhone still doesn’t turn on, or if it gets stuck during start up, see the Apple Support article If your iPhone, iPad, or iPod touch won’t turn on or is frozen.
...
copied from: Force restart iPhone - Apple Support