Your boiler has stopped working because it cannot correctly measure the temperature of the water returning to the unit due to a faulty sensor or connection.

Return–NTC: NTC broken, NTC cable broken, Defective connection at NTC, Defective connection at electronics | Cause: Return–NTC cable defective/broken NTC faulty

What causes this fault?

This fault occurs when the boiler's internal computer loses communication with the sensor that monitors the temperature of the water coming back from your radiators. The most common reason is that the sensor itself has failed due to age or electrical failure, though it can also be caused by a loose connection or damaged wiring. Because the boiler can no longer safely monitor heat levels, it shuts down as a protective measure.