[JimKrieger]

Epilog. Removed all screws anchoring furnace to floor and was able to partially extract the unit without disconnecting gas or electricity. Turned on the thermostat and the furnace ignited and worked normally. Guessed that the movement may have jarred a failing sail switch back into service, so replaced that. Did some cleaning, re-mounted the unit and it has continued to work. Apparently on my furnace, a failed sail switch, electrically or from dirt, still allows the fan and gas solenoid to work and only prevents power to the igniter. Thanks to everyone for all the helpful suggestions.