Try to hard restart your iPhone by clicking the volume up button then the volume down button then hold the side button for 10-15 seconds. If that doesn't work then put your iPhone into recovery mode by connecting your iPhone to a computer that has iTunes open and you are signed into your account, once you've done that, then click the volume up button, then click volume down, then hold the side button until you see the computer glyph on your iPhone screen. If you need any extra help then visit this link, or contact Apple Support directly.