Powerpoint loses all slide modifications after crash and reverts to original slideshow

[google]

I was building a PowerPoint based on a prior slideshow when for no
apparent reason the program crashed. No matter, I thought, as I had
saved the new slideshow to a different filename and had not only
relied on autosave, but had hit command-s a number of times during the
build. I was shocked upon restart to open the file (by the NEW
filename) from the "recent items" section of the Apple menu and be
presented with the original file with all new material gone. Why did
this happen, and is there any way to recover it?

All suggestions gratefully received.