Flattened a file and saved it

To my knowledge, there's only one thing you can do.

If you've saved it (but haven't closed the file)... you can simply go to history, find the layer above Flatten Image, and click on that.

In my example image, it's the layer with the star.  Just click on that layer in History, and it'll take you back to the point right before flattening. (Any work you did after that will have to be re-done.)

If you've already saved it AND closed it, you're out of luck.