iPhone xr auto shut down won’t open back

Force restart iPhone - Apple Support ⬅︎ try this and if still the same put it in Recovery Mode...

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1. Press and quickly release the volume up button.
2. Press and quickly release the volume down button.
3. Press and hold the side button.
4. 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...⬆︎⬆︎⬆︎short instructions ⬇︎⬇︎⬇︎

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...