Okay, so I was working on this map of a massive city that came to 5000px wide at full size. Along the way I've been resizing the image down to 1800px so I can take snapshots of the progress that are just big enough to make out the details. My problem is that after my last resize for the latest snapshot, I forgot to undo it to get it back to the full size and I accidentally saved over it. I did enough small edits by the time I caught it that I couldn't undo enough to get back to that point.

Does anyone know of any way to recover a previously saved version? I'm on a Macbook Pro and I haven't turned my computer off or shut down Photoshop yet.