When an app becomes unresponsive, you can reset it. To close the app, open the App Switcher and swipe the screenshot of the app upwards to close it.
This forces the app to quit; wait a few seconds, open it again, and hopefully it will be back to normal.
For the record, though this is a useful technique for dealing with misbehaving apps, it’s not necessary to manually close apps at any other time. Contrary to popular belief, it won’t speed up your device or extend battery life; it will simply make apps take longer to reopen next time you use them.
Force bad apps to restart
If an app stops responding completely, you won’t be able to force quit it through the App Switcher as described above.
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 restarts the current app.