It doesn’t happen very often, but sometimes an app might crash, freeze or become unusable. That might mean when you tap buttons nothing happens, or maybe a photo will be stuck on the screen. Thankfully it’s easy to fix this problem in just a few steps.
To force-quit an app that’s frozen, double-tap the Home button under the screen, then tap and hold on the icon of the app that has crashed. After a second you’ll start to see it wobble. When it does, tap the small X button that’s appeared. The app will now be completely shut down, so when you re-open it will fully work as intended.