Dec 13, 2016 was when it last did this.
Was still on the clock so I just turned off power to the furnace and then turned it on again; that sufficed to get it running to bring the temp up. Once work was over, it had dropped to 68, so I went down there, pulled the thermocouple, cleaned the carbon off it, and put it back in. Hopefully that'll hold it for a while.
Like maybe another two years.
...but of course after spending my day in a cooling house, now that it's 70 in here again, it feels too hot. Heh.