Thanks. It works. For now, at least.
The error I got was AD1133, which I found out means something like "We're no longer operating in your area, so we closed the server you're trying to connect to. Good luck and goodbye".
I found a guide to do a hard reset instead:
http://heeed.net/resetting-a-nook-simple-touch/
It's a bit of an adventure, but it works perfectly. I'm no longer able to connect to B&N, but that became irrelevant quite some time ago anyway. I put the SD back in and formatted it from the device. And connected to PC. No errors. I can also transfer books from Calibre again and read them on my device. I have no idea about what went wrong yesterday. Now I'll see if I can do the same for my wife's Nook.
Thanks so much for the help and support, guys. I was just about ready to give up here.