Interesting. This whole set up is pretty ancient and I would do well
to have it serviced soon, I think. Meanwhile, I gently cleaned the
flame sensor, checked for obstructions and Violá . . . heat! So far so
good. A new thermostat may yet be in my future, tho! Thank you!

Don't tell anyone from HVAC that you fixed it yourself or you may be
threatened with scenarios of your house blowing up or being overcome
by toxic fumes.

They would rather have you pay them first before they get the chance
at blowing up your house.