I have a Lennox air/heat system about 20 yrs old upgraded to electronic lighting. I replaced the filters and bumped the cover that houses the wiring to the system (power cord form the wall to the transformer onto the various sub systems) After I closed the panel, I noticed my thermostadt lighting display not working. (It is powered by the voltage from the transformer---no batteries) That is, the thermostadt (all elcetronic )was "dead". I replaced it with a known working one and still the same---no display. I checked the wiring, the fuse to the thermosadt from the transformer, and replaced the transformer. I tested the old one (an ohms check) and it was OK anyway. I used a digital multimeter to follow the 120v form the wall receptacle to the "fail safe" switch (a switch that cuts power to the system if the panel is removed for filter replacing and will not continue the current until it is put back on). When the switch is pressed to continue the flow, the 120 volts continue to the relay and from the relay to the new transformer. The transformer is grounded as the old one was. When I put my test leads to the 24volt output of the transformer, there is no voltage at all (supposed to be 24volts) from either of the 2 screws no matter how I ground the screws. One of these screws (load) goes to the thermostadt and the other to the system. All wires were checked and are tight--no breakage. The system (furnace) worked until I replaced the filters and when I bumped the cover covering the wires. There are no sensors at or near the filters as this is an old furnace about 20 yrs old. If it makes a difference, it is a natural gas unit. :confused:

And you are certain that there is 120V going into the transformer,and nothing coming out,, sounds like another blown transformer, maybe something is loose shorting out the transformer,, are you certain the door button is closed when you check the voltage at the transformer,, double, tripple check and get back to us