Arrived on site and was shown the thermostat and unit that were not responding. Onsite crew informed me that earlier today they attempted to cool the space but the thermostat on the wall was completely blank and unresponsive. I checked and confirmed tat the back plate was not receiving the proper voltage. Onsite management opened up a utility closet and sowed me one electrical panel. I confirmed nothing was popped on it. I saw a larger second panel and identified a single breaker was popped. I reset the breaker and went back to check voltage. I was now getting the needed power. I replaced the thermostat back on the wall and the unit it immediately kicked on. I let management know where they could find and identify the tripped breaker and they let me know that in 4 yrs they had never even opened the second panel. The system is now up and running and cooling on departure.