First, please make sure that you are running the latest game version by visiting your mobile app store. If you're not sure if you have the latest version, please click here for steps on how to update your apps.  
1. Press the home button two time quickly. You will see small previews of your recently used apps.
2. Swipe your finger up on each of the apps you see to close them. Further help with this can be found here.
3. Press and hold the Sleep/Wake button until the red slider appears.
4. Drag the slider to turn your device completely off.
5. After the device turns off, press and hold the Sleep/Wake button again until you see the Apple logo.
6. Reopen the game and it will prompt you to reconnect to Facebook. If you had your game connected to Facebook, reconnect.
We recommend you try these steps every once in a while to lower the frequency of the game freezing! If your game still freezes after you tried this for the first time, it may require that you close the app fully and turn it back on a second time.