Hard reset

The procedure differs, depending upon which model iPhone you are using.

If you are using an older iPhone or an SE model, one that has a Home button, press and hold the Home button while pressing and holding the Lock button. Keep both depressed as the "Slide to shut down" message comes and goes and only release them when you see the Apple logo in the center of the screen.

If you are using a newer iPhone model that uses FaceID, press and release the Volume Up button, then press and release the Volume Down button and then press and hold the Lock button. Keep the Lock button depressed as the "Slide to shut down" message comes and goes and only release it when you see the Apple logo in the center of the screen.

Good luck.