Your boiler has switched itself off as a safety precaution because it has either detected a fault or has become too hot.

Non - Volatile Lockout / Reset O/H or in 'standby' mode

What causes this fault?

This fault occurs when the internal safety sensors detect that the water inside the unit has reached a dangerously high temperature or if the system has failed to ignite properly. The most common reason is a lack of water circulation, often caused by a failing pump or a blockage in the pipework that prevents heat from moving away from the boiler. It can also be triggered by a faulty control board incorrectly reading the system's temperature.