My heat is not working. The fan on the ac unit does not come on. What could be the problem? the emerency heat switch on the thermostat goes on and heat comes out. Is it safe to use the emerency setting

A heat pump is an Air conditioning system that is reversible... in the summer it takes the heat out of the house and gets rid of it outside... in the winter it takes heat from the air outside (much harder to do) and gets rid of it in the house...

But it don't work well below 30F outside... so you have electric heat (AKA emergency heat).

Unless its bitter cold outside... too cold to work... then its time to call someone to check it out... electric heat will be expensive.

Check the circuit breaker for the outdoor unit. It is also possible the outdoor unit is shut down via an outdoor thermostat. Sometimes installers will connect an outdoor thermostat to shut down the unit if the temperature drops below some predetermined point. The idea of such a thermostat is to shut down the unit when the cost of running it exceeds the cost of heating with the electric resistance heat (auxiliary/emergency heat) installed in the air handler.