If you have Google Drive cloud save file syncing enabled, in rare occasions (especially if you are playing on multiple devices, or don't always have a data or Wi-Fi connection), it is possible for your most recent save file to be overwritten by an older save file.
Unfortunately, once lost there's no way for you to get that save data back, but there are a couple solutions you can use to help prevent this going forward.
- You can make multiple manual save files from time to time just in case one save file is lost due to an issue like this. You can do this by tapping and holding on the popup text that appears when you go near a save point, and then selecting a new save slot.
- If you don't use the cloud syncing feature for save files, you can disable it by going into your Google+ settings -> your account -> Apps with Google+ Sign-in -> The Bard's Tale -> Disconnect.