Your boiler has stopped providing hot water because the internal sensor that measures water temperature is broken or disconnected.

Cylinder temperature sensor/comfort sensor/loading sensor lead break

What causes this fault?

This fault occurs when the boiler's control system loses communication with the sensor responsible for measuring your hot water temperature. The most common reason is that the sensor has suffered an internal electrical failure or the connection wires have become damaged or loose. Because the boiler no longer knows how hot the water is, it shuts down the burner as a safety precaution to prevent scalding.