Your heat pump has stopped working because a sensor that monitors the temperature of the cooling liquid returning to the main unit has developed a fault.

Compressor Refrigerant Return Thermistor (Th) Error (E9)

What causes this fault?

This fault occurs when the system's brain loses track of the temperature of the refrigerant liquid as it flows back into the compressor. The most common reason is a failed electrical sensor that has either burnt out or is sending back scrambled readings that don't make sense. It can also be caused by a loose connection or a damaged wire between that sensor and the main control board.