My heat pump turns on and runns for a while and cuts off like it is heating, but it does not heat up the house. What could be wrong with it? The tempoutside has been down in the teens and lower would that have anything to do with it?

Does the air handler have a additional heating coil(electric)? A heat pump alone will typically not be enough to heat a house or condo when temps get down to the teens.

Dose your air handler have the auxiliary (also called Emergency) heat strips installed? A heat pump alone once the outdoor ambient temperature drops below freezing the heat pump tends to struggle,that's when AUX kicks in. also you could have a refrigerant leak.