Iff you haven't resaved since, you may find a backup copy of the prior version of the .pz3 in your directory. Can't remember what it's renamed to, but it was obvious. This is not deleted until the current save is complete, so it should be there. If you find it, rename it now so it isn't overwritten. Obviously it'll be out of date, but I've had some luck in editing the corrupt one in a text editor (trim it off to a point where it'll at least load a partial scene), and then saving out parts of the new scene, e.g. characters, poses, props, and then loading into the renamed old saved one... As I say, sometimes.