How do I get old accounts back from accidentally getting a new account?

Game progress is saved through Facebook so once in the game, go to your settings and select Log Into Facebook. You should get a pop up saying you already have a saved game. It will then ask you if you want to connect to the level bla bla game or lower level game. Just make sure you select the right game.

If you have already done this and selected the lower level game by mistake, there is nothing else that can be done because the data is lost. Once it is over written, it cannot be "rolled back". Sorry.