First, make sure you are on the latest iOS update. Update to the latest iOS to get the fixes for memory issues.
You maybe are running into a common device issue that is corrected by restarting your device. Press on the power button for 3-5 seconds until you see the bar to power it off. Slide the bar and power the device down then power it back on, tap back into the application and see if that corrects the problem. The restart will usually clear memory issues on the device.
If you are still having issues after the power cycle, then close the game by force closing the application. Press the 'Home' button twice really quickly. The multitasking screen will pop up and allow you to swipe your finger from the bottom to top to remove the different applications from multitasking. If there are many applications in there, you may want to clear them out as well. Then press the middle button to get out of the multitasking mode and start the game again.