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.

Swipe the app screenshot upwards to close it completely

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.