This is likely due to your Internet connection dropping while playing the game. Not to worry though as it can be resolved as follows:
  1. Check that your device has a fast and stable internet connection here. The minimum recommended download speed is 3 Mbps, upload speed is 1 Mbps and a ping rate must be under 150 ms.
  2. If you're using mobile internet, try switching to Wi-Fi.
  3. Turn your device off and on.
If you are connected to Facebook in the game, clearing cache in Safari should fix it:
  1. Open Settings
  2. Scroll down and tap on Safari
  3. Scroll down again and tap on "Clear History and Website Data"
  4. Tap on "Clear History and Data" again
  5. Force close and reload the game before reconnecting to Facebook in the game