Follow the steps below to force an app to close:
If you are using an iOS 7, 8 or 9 device:
- From the Home screen, click the Home button twice.
- Tap and swipe up with the app.
- Once you have force closed the app(s) from this screen press the Home button once more.
- Now try opening the app again.
More information on multitasking and apps is available at http://support.apple.com/kb/ht5137