All I can suggest is that next time you "save as" instead of just "save", that way you can save "seperate" files all the ways along, (changing the name or numbering them so they will save as a different file), instead of just "one" in case this happens so you can go back to at least a prior save. Takes up a lot of room but this can be temporary till your finished and then once your done you can delete all your composites.