Your boiler has shut down because it has detected it is getting too hot, triggering a safety switch to prevent damage.

Overheat - Normally associated with either poor or lack of primary water circulation

What causes this fault?

This fault occurs when the water inside the boiler exceeds a safe temperature, causing the safety limit thermostat to trip and shut everything down. The most common reasons are a pump failure preventing water from circulating or a buildup of limescale and sludge blocking the internal heat exchanger. It can also happen if there is trapped air in the system or if all your radiator valves are closed at once.