If your game started from the beginning, and if you did not reinstall your phone's OS or manually deleted the game data, there are couple reasons for that.
First, if you are playing as a guest, your game data is saved to your phone. As a guest, your game progress would rarely restart from the beginning. Unless you reinstall your OS or manually delete the game data from your phone's settings, your progress will be saved to the phone's memory and always available.
Now, if your game restarted from the beginning, it's is very likely that you have connected the game to a service like Facebook or LINE in the game's settings. This is by design because our game only allows one account to be connected. All you have to do is, go back into the game's settings and connect it to the same service with the same account. If you have multiple Facebook or Google accounts, please recall the one you used and connect with it.