Sometimes apps, especially games, can get stuck or crash while you are using them. This isn't a common occurrence on iOS devices but it does happen from time to time, and it can be annoying. Luckily there is an easy solution to the problem. To restart/reboot a crashed app just double tap your Home button to open the App Switcher. The App Switcher will display all the apps currently open on your device. Locate the window for the stuck app then swipe up to swipe it away. This will reboot the app and allow you to reopen it and start using it again.
How do I restart an unresponsive / crashed app?