1) First, you can try force-closing the app and starting it again.
If you're on an ANDROID device:
Please go to Settings > Applications OR Task Manager > Running. You can tap "End" to close the app. This will completely close the app. This setting may be different depending on your device model.
If you're on an APPLE device:
To manually quit the app on an Apple device supporting multitasking, first press the phone's home button to get to your home screen. Then hit your home button twice, and a bar should appear at the bottom of the screen. Tap and hold the game's icon there until it wiggles, and hit the red "-" button that shows up. (Make sure you do this with the icon at the bottom of your screen and not the icon on your actual home screen. If you get a message about deleting the app, you're looking at the wrong icon.)
If you are using an Apple device that is running iOS7, then it will be a little different. You will need to press the home button to get to your home screen. Once you have done that, you can tap the home button twice, and you will see the screen shift. You should now be seeing multiple icons at the bottom, with an image of what your app would be showing if you were currently running that app. Tap and hold on the image of the app screen, and slide your finger towards the top of the screen. You will see the image slide off the screen, and the icon will be removed from your list. Once you have manually quit your apps, tap the home button again, and you will be brought back to your home screen.
2) If this doesn't help, or your device doesn't support multitasking, turn off (power down) your device, turn it back on, and restart the app.
3) Try deleting the application, then download the latest version of the App and reinstall it. This will not affect or remove your game data, since it is saved on our servers.