Phone screen is black with the loading symbol

Try a Forced Restart on the phone. Follow the instructions very carefully, as this is a bit tricky. (Hint.....Press and release the Volume Up button quickly, do the same with the Volume down button.....and ignore the "slider" when it appears and keep holding the Side button in until the Apple logo appears. This might take another 15-20 seconds.)

Force restart iPhone - Apple Support

There are only 2 possible results when you perform a Forced Restart:

1) The phone returns to normal operation.

However, if the phone will still not restart correctly....

2) There is a hardware problem with the phone that will need attention at the Apple Store (make an appointment) or take the phone to an authorized Apple Service shop.

You can find a list of authorized service shops in your area using the link just below:

Find Locations