If you take a break from playing the game in the middle of a level to do other things (checking your email, feed the dog, and so on), your device may secretly force the app to stop to conserve memory and battery life. 

When this happens, upon returning to the game it will restart, and your progress from the middle of the level will be lost, and the life you used will be gone. 

The duration of time before the device does this depends on the device, and we have no control over this. For this reason, we recommend you finish up anything important you were doing in the game before switching to another app or putting your device to sleep.