Air or heat doesn't work. Fan blowes constently if left on at breaker. What could be the problem? 1 UNIT if I need to replace?

When you lose the 24 volt control voltage (transformer) that's what will happen, the fan runs constantly. Check for a blown fuse on the printed circuit board in the furnace or air handler, if it is bad you may have a short in the thermostat wiring or a burnt control device. The fuse will be an automotive type. Look outside at the thermostat wires and see if they are bare or touching.