Have you reinstalled the game? Changed your device? Are you playing on two devices? It may happen that you need to restore or sync your game progress.
iOS
As you play, the game progress is automatically stored in iCloud (assuming the iCloud account is configured correctly). When you reinstall the game and log in to iCloud again, your progress will be automatically restored.
Android
As you play, the game progress is automatically stored in Google Play. When you reinstall the game and log into your Google Play account again, your progress will be automatically restored.