Hi
First thing to try is a forced restart:
* 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 Apple logo. (KEEP it pressed until the apple Logo appears)
If that doesn't clear it, see:
https://support.apple.com/en-gb/HT201412
But for that you will need a mac or PC. You will either have to find someone who will let you use theirs, or take the phone to a repair shop and ask them to do the reset for you.
Alternatively if you can't do that, the most likely cause (but not guaranteed) of the problem is a worn out battery, especially if left discharged in storage. You could just try having the battery replaced. But then if you were doing that, you could just ask the repair facility to try the restore first.