There are two occasions that will result in your game starting from the beginning or if you find your previous progress has disappeared.
1) If you are playing as a guest and you reinstall your phone OS or manually delete game files, your game data will be deleted and won’t be recoverable, resulting in starting over.
2) If you have connected your game to a service like Google or Apple, then your game will start from the beginning if you reinstall or disconnect any of those service in the game. However, because the data is attached to the account you connected with before, you can recover your game progress by connecting to the same account you used before. Recall the account you used and connect again.