We have a heat pump for heating and cooling. Earlier today the power to the Nest thermostat went off. When the power was restored, the air conditioner did not kick back on. We have checked the circuit breaker to the air conditioner and checked the battery for the thermostat and reset the thermostat system. At this point, the thermostat says the air conditioning is on, the internal fan is still running, but the outside air conditioning unit is not turning on. When tested, the thermostat states T02 - single cooling system problem.