Your boiler has shut down because a sensor has detected that the exhaust gases are becoming too hot, which is a safety feature to prevent damage.

Flue negative thermistor coefficient (NTC) sensor operated

What causes this fault?

This fault occurs when the internal temperature of the exhaust gases exceeds a safe limit, triggering a safety shut-off to protect the boiler. The most common reasons are a build-up of debris or limescale inside the main heat exchanger which prevents heat from transferring to the water, or damaged internal insulation that is allowing too much heat to reach the sensor.