If quitting the app (pressing the home button) and loading it again doesn't fix the problem, the next thing to try would be to:
On the home screen, press the home button twice in quick succession to bring up the multitasking bar.
Hold down your finger on the app until the red quit button shows up, and then tap on it to quit the app.
Try launching the game again.
If this doesn't work, the next step would be to try restarting the device. Hold down the sleep button until you are prompted to "slide to power off" and do that. Wait until it turns off, then hold down the sleep button for a few more seconds to turn it back on.
If the device is totally frozen and you can't get back to the home screen, hold down the home and sleep buttons together until the device restarts and the Apple logo shows up, then let go and wait for it to load.
Hopefully at least one of these will sort things for you.
It did the same thing on my iPad 3. After trying all of the above, I finally had to delete it and download again. Not a good solution according to my nephew, because he lost all of his previous scores.