Your boiler has sensed a flame inside when there shouldn't be one, so it has shut down as a safety precaution.

Safety switch off flame simulation

What causes this fault?

This fault occurs when the boiler's internal sensors continue to detect a flame even after the gas has been turned off and the burners should be extinguished. The most common reason is a malfunctioning flame sensor or a faulty gas valve that is leaking fuel into the combustion chamber, causing the boiler to shut down for your safety.