So I got up this morning, and it was really cold in the house. The furnace was flashing an error code.
I called the furnace guy, and he found my house a bit more than an hour later, and saw that there was a visible hole in the igniter. (Once he pointed it out I could see it.) He tested it to be sure, and it was bad; he installed a new one; and now I have heat again. Cost a little more than I would like but not enough to really be a problem