If the game hangs up / stops responding / start behaving unpredictably, in most cases relaunching it will fix the problem.
Close the app completely from your phone’s task manager. To access it double tap home button on the iPhone or use the dedicated “task manager” button on your Android Phone (square button in most cases). Swipe to close the game and then run it again.
If you’re still having problems please try turning off and on or resetting your device.