When an app becomes unresponsive, you can reset it. To close the app open the multitasking view by quickly double-pressing the Home button. From here swipe the screenshot of the app upwards to close it.
Now close the view by pressing the Home button, then open the app again and it should be working.
Force bad apps to restart
If an app stops responding completely, you won’t be able to quit by pressing the Home button, as per usual.
Instead, press and hold the On/Off button until the slide to power off message appears and then let go.
Then press the Home button for around six-seconds. This forcibly quits the current app and returns you to the Home screen. You can then reopen the app by tapping on its icon.