Document is missing pages/content?

I was working on a 50-60 page document for a couple of hours. I was making some tweaks to it (just moving around some text) and all of a sudden my document just loses 10-15 pages.

Literally. It's like the pages just disappeared from the document. Mind you, I save often. I tried undoing whatever I had done (which really wasn't anything drastic, just moving around some text), but no dice. Hours of work just gone.

What happened? Is it possible to recover it?