Pressing 'Delete' whilst in Layer Box

I've done this a few times now whilst trying to remember no to - instead of using the 'undo' on the top bar to step back I've pressed delete whilst I have a layer active .. and everything disappears!  By fiddling about I can get the catalogue back but the image I was working on seems to disappear into the ether .. how can I quickly remedy this ??  .. any suggestions/advice appreciated!