Your boiler has shut down because a safety component has detected that the internal temperature reached a level high enough to potentially damage the system.

Thermal fuse. Replace the heat exchanger

What causes this fault?

This fault occurs when a safety sensor detects that the heat exchanger has reached an dangerously high temperature, often due to a lack of water flow or a buildup of debris. Because this component is designed to permanently break to protect the boiler from melting or catching fire, it means the system has suffered a significant overheat event. The most common reasons are a failed pump, a severe blockage in the pipework, or a lack of water in the system.