I have a therm-o-matic T114 gas furnace the furnace stopped working and will not restart We replaced the thermostat upstairs and it still will not restart, it is like the mercury switch does not function

Mercury switch in what? How old? Voltage at the gas valve? This has a standing pilot?Most of these date to the 60's. While they still may be good, you are certainly throwing money away with a 60% AFUE heater.