get original photo back after cropping

It depends on what Photo editing app you used.

If you didn't save already you can undo.

If you are using Aperture 3, just hit the letter M for master and you should get your original photo or go to Photo on the top bar and click on it then in the frop down menu select remove adjustments (I think that's the option-I am not at my computer at home to check the precise wording).