|Heat Pump is Frozen
Heat pumps can and will freeze on cold humid days. This is a known part of their operation, and because this happens, the manufacturers have built a defrost cycle into your heat pump, the same as a refrigerator or freezer. The defroster is on a timer and will operate when needed. If it fails to operate properly, it may be due to severe icing, or it may be malfunctioning.
A heat pump had two stages, the secondary heat source is electric or auxiliary. Switching to auxiliary for a few hours will turn off the heat pump condenser and allow it to cycle thru a few defrost cycles. If this does not remedy the situation you will probably need professional assistance.