If an app is unresponsive but the system seems to respond to when you press any button, you can force the app to close:
Step 1: Double-click the Home button.
Step 2: Swipe left or right until you have located the app you wish to close
Step 3. Swipe the app up to close it.
Note: It's typically unnecessary to force an application to close unless the app is unresponsive. You can view all recently used apps by double-clicking the Home button. These apps are not open; they are in a suspended state.
If you close an app that has been configured to receive background notifications, it will no longer receive them until it has been reopened.