DIAGNOSIS The thermostat has detected an open circuit in the heating call wiring, specifically the O/B reversing valve or auxiliary heat (W2/AUX) circuit for a heat pump system.
FIX Go to thermostat settings > equipment. Confirm your system type is correctly set to "Heat Pump". Then, in the same menu, verify the "O/B Reversing Valve" setting matches your unit. For most brands (like Carrier, Bryant), set to "O". For Rheem, Ruud, set to "B".
IF THAT DIDN'T WORK The issue is likely at the outdoor unit. A tripped high-pressure switch, low refrigerant, or a faulty contactor can prevent the heat pump from responding. This requires an HVAC technician to diagnose.
HEADS UP The "no power to heat pump" alert specifically means the thermostat is sending 24V but not detecting a completed circuit. It will not detect a blown fuse at the outdoor disconnect.