A. Check for a dirty filter, is the power switch on, is the circuit breaker tripped, is the thermostat set to heat or cool and the proper temperature?

A. This is a possibility.  Thanks to innovations in tecnology, such as the hyper-heating inverter, heat pumps can now keep a home toasty warm even if outdoor temperatures drop to 0^ºF!  That's a big improvement over the heat pumps of the past, which became ineffective when outdoor temperatures reached the freezing point (32ºF).