Unlike most other photo editors the XPE allows you to un-crop or restore the cropped areas at any time even for cropped pictures that you've opened, that were cropped in the past.
To do this just open a picture that has been cropped (the title bar shows whether the picture has been cropped or not), go into the crop tool and select the Un-crop button. The cropped areas are restored—you can now adjust and re-crop if you require or click the Clear button restore the whole image back to its original.
See also
'Magic' undo