Your boiler has switched itself off because the internal temperature sensors are giving conflicting or unusual readings, meaning it cannot safely regulate the heat.

Lockout Burner Mains - Automatic Flow Thermistor Fault (Thermistors reading outside of limits or difference between readings outside of limits)

What causes this fault?

This fault occurs when the sensors responsible for monitoring the temperature of the water leaving the boiler fail to report consistent data. The most common reason is that the internal components of the sensor have degraded over time, though it can also be caused by loose wiring or a communication error within the main control board.