First thing to do is to make sure you record your current Game ID so you do not lose any new progress you have made if your old account can't be recovered. Your Game ID can be found on the screen when the game loads or in the Settings menu.
Sometimes your account can be lost because you were disconnected from Facebook or Game Center, or you loaded the wrong game. To fix that, go to the Settings menu via the cog at the top right of the screen, tap About, then Connect, then Disconnect. That should take you back to the main screen and if you tap Log In you will be able to log back in and select from all the game accounts you had linked by either Game Center or Facebook. So if you linked your old account, it should be there for you to select.
If that does not locate your account, please contact our support team in the game by going to Settings, tapping FAQs and Support, then Yes, then taping Submit Request. If you know the Game ID of your lost account, be sure to include that in the message.