Your boiler has switched itself off because the water inside it is heating up much faster than it is moving away, preventing it from working as intended.

System out of operation (in case of a temperature gradient higher than the reference value). Make sure that there is circulation in the heat exchanger; check the open position of the valves, check the circulating pump operation). In such a case, the fault can be automatically remedied after the temperature gradient become ≤5°C.