Click on the Home Button on the bottom of the device and then slide to unlock the screen.
When you're at the main screen double click the Home Button.
Hold down an app until a red arrow comes up by the app and then press the red arrows by the apps to make them stop running on your device.
Now tap on the Home Screen to access the Home Screen.
Now restart your device by holding the power button on the top and when it is done shutting down turn it back on by holding the power button down again.
Now when the the screen goes back to the Lock Screen slide to unlock the screen.
Now go into the app that was not working.
Now the app works.