Force restart iPhone - Apple Support ⬅︎ TRY THIS, and if it does not work try Recovery Mode because if you can't get it to start properly then you have to have it serviced by Apple which you might not want to do and might not be an option right now...
The other issue is your battery so this may not work but I sure hope it does...
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Force restart iPhone 7 or iPhone 7 Plus
- Press and hold both the volume down button and the Sleep/Wake button.
- 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.
Recovery Mode... the full info is in the above link...⬆︎ the short info is 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.
iPhone 7, iPhone 7 Plus, and iPod touch (7th generation): 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 Recovery Mode...Once you get into Recovery Mode then your phone will be recognized...