Sometimes apps, especially games, become unresponsive even on iOS devices. If this happens on your iPad or any iOS device, just double tap the Home button to open the App Switcher. The App Switcher will display all your currently opened apps. Just select the panel of the app that is stuck and swipe up to reboot it. Now open the app again and it should be unstuck.
You can also use a four finger swipe to open the App Switcher if your iPad Multitasking Gestures are enabled. You can enable Multitasking Gestures in the Settings app under General.