I came home and it was 60F (15C) degrees in the house... and it's not supposed to be quite that chilly!
So, I checked, and the natural gas pilot lights are out under the boiler. I try to light what seems to be the primary pilot light. By following the steps I can get it lit, but it won't stay lit when I ease my finger off the pilot-lighting button.
I've seen this before and recalled the diagnosis - the thermocouple has gone bad. Basically, it's a safety feature. You need safety features with natural gas, because without them you can literally blow your house to smithereens.
My wife called up the guy who put the boiler in, and he was busy on another job, but he offered me some advice on what kind of replacement thermocouple to buy. So we bought one, and I put it in, and we have central heat again. So I don't have to worry about the water pipes freezing tonight.
I got it working fine and dandy
even though I'm not that handy.