Your boiler has detected that it is running too hot and has automatically shut itself down to prevent damage.

high limit thermostat/burner

What causes this fault?

This fault occurs when the boiler's control system encounters a momentary communication error or an unstable electrical signal. It is usually caused by the internal computer board failing to receive a clear confirmation that the burner has lit correctly, or a brief glitch in the electronic sequences. The boiler will typically attempt to clear this itself before deciding whether to stop completely.