iPhone will not turn on

Force restart iPhone - Apple Support...Try this...

Force restart iPhone 7 or iPhone 7 Plus

1. Press and hold both the volume down button and the Sleep/Wake button.
2. When the Apple logo appears, release both buttons.

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.

If the above does not work again, try Recovery Mode, full info is in the above link...short info see below..

Connect your device to a computer. On a Mac with macOS Catalina 10.15, open Finder. On a Mac with macOS Mojave 10.14 or earlier, or on a PC, open iTunes.

Press and hold the Top (or Side) and Volume Down buttons at the same time. Keep holding them until you see the recovery mode screen.

It sometimes takes a few tries to get it right and when that happens only then will your phone be recognized.