If the fan is blowing with no heat, it is an indication you have an error code. Check the control board for a blinking light. If you find one, look on the wiring diagram that is pasted on the reverse side of one of the panels for a diagnostic chart. It will give you a starting point where to look for the issue. You can always jump the red and white wires on the thermostat and if the furnace starts, it's a bad thermostat. If not, look to the blinking light.