I have a Lennox WhisperHeat (~1991). When I moved into the house in 2001, I installed a new thermostat which has been working fine- battery powered, electronic. In the last few weeks, I have had two instances where I have come home to find the furnace not working. When I cycle the thermostat or the circuit breaker off and back on, the blower comes on and the pilot ignites, but the burners don't ignite.
This has all been done with the safety panels in place, so there is not an issue with safety switches. Both times I have been able to get the furnace working again by cycling the circuit breaker on and off several times.