Board Thread:Questions and Answers/@comment-28475652-20160517055230/@comment-24659969-20160517061830

Battle cats does not support automatic Google's Cloud save / backup. Instead, PONOS as the developer, provided a save data transfer (NOT backup, ok?) service using own server. To initialize the data transfer, on old save you have to go to main menu Setting > Save Data Transfer > Confirm. After doing so, the save data in the device will be uploaded to server which kept for a period of time and the current save data in the device will be locked until cancelled or the timer expired which returns the save data back OR claimed using other app client which deletes the old save data.

Tldr: If you forgot to initialize the save data transfer procedure I bolded above, your save data pretty much gone bye bye already.