Now at least I know what I did. It doesn't make it any easier, and the file is not recoverable, but at least I know. Note to self - NEVER try new software when finishing up a book.
So... I've learned that in order for Dropbox to save a file, you have to be working out of the Dropbox folder on your desktop. Not out of the Word folder, but the Dropbox one. I wasn't. That's why, when I emailed the file to myself, thinking I was sending the most recent file, I emailed from Dropbox and of course it sent the one I worked on there last, which was 8-29.
When I finished my book, jubilant - I shouted to my husband I was done. 72,211 words, 345 pages. I saved it, promptly emailed it to myself (not noticing I sent an old version), and emailed by BFF Anna to tell her I had finished. I then thought of something, went into Dropbox, opened the (wrong) file, and then hit save. To be fair, the warning did come up. But I, still thinking I was in the most recent version, hit save anyway.
And thus overrode (and destroyed) the old file. It's gone. Not recoverable. I have tried everything, including calling a computer repair shop.
My beautiful ending - disappeared. I have to try to recreate that, plus all the explanations and stuff that I wove back through the story. I am now at 321 pages, no last chapter yet. Very sad. Still sick. But a little bit wiser.
Oh, I wish I had waited to try Dropbox until after I'd finished the book. You lose the first chapter of a new book, no big deal. But the ending? The tying up all the threads - the suspense, the romance, the secondary character's romance. Aiiiiiyyyyyyy.
So now you know. A word to the wise. And that's how I'm spending my holiday weekend. Rewriting and praying I can remember what I did and didn't do.