Check the heat pump that is responsible for heating the domestic water for an alarm. If there is an alarm, refer to the manufacturer’s manual for resetting instructions and/or other troubleshooting suggestions.
Contact a qualified service technician for troubleshooting and repair if needed. You can contact a technician by reviewing the sticker attached to the heat pump to find the manufacturer, make and model of the heat pump and then searching online to find service technicians with experience working with the equipment.