Force restart iPhone - Apple Support ⬅︎⬅︎ try this even if already tried...
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- 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.
If the above does not work try Recovery Mode...see full instructions in the above link...
- 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.
- iPhone 8 or later: Press and quickly release the Volume Up button. Press and quickly release the Volume Down button. Then, press and hold the Side button until you see the recovery mode screen.