I knew, of course, that I should back up regularly and that my usual practice of periodic back-ups wasn't sufficient. I also had the sense that another back-up was past due. But since there were no signs of imminent collapse, I didn't get to it in time.
On the positive side, I only lost one (very productive) day from the notes I was taking in Zotero, and only 3 weeks of everything else. And it wasn't the worst time to lose data: It wasn't mid-semester and I wasn't in the thick of drafting an essay.
Still, one week later, I am still trying to remember, record, and retrace my steps as well as I can, and it will be some time before a permanent replacement will be in hand and ready for work.
Lesson learned, I hope. In future, my computer will be set to back up automatically to an external hard drive, and/or to my institution's server, and/or to the cloud. I recommend you do the same.