My downstairs unit will not turn on when the thermostat clicks the heat on. The ac turns on, abd the upstairs unit works fine. Why would the unit not work for heat?

Would help if we knew what kind of equipment you had, gas,electric,oil, furnace, forced air,heat pump, also name brand,, also have you checked the breakers?do you have 2 units or just one? So many questions need many amswers