I was a foolish person and didn't realize that I was editing a different notebook than I intended to. As a result, I lost a couple hours' worth of work and get to rework a problem that was pretty well settled.
Obviously the first/best solution would be for me to not be foolish. Barring that possibility, having the ability to view version histories for notebooks (and cards?), and to be able to restore to previous versions would be extremely helpful in case anyone else makes similar mistakes down the road.