Force restart iPhone - Apple Support ⬅︎ try this one more time and if still the same try to put it in Recovery Mode...
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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.
If your iPhone, iPad, or iPod touch won't turn on or is frozen - Apple Support
Try Recovery Mode...⬇︎⬇︎⬇︎Full instructions in 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.
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.
It may take a few tries to get into Recovery Mode but when you do then the phone will be recognized...
If still the same issue after trying the above click "Contact Support" top right corner of this page for further help from Apple...