Your boiler is temporarily preventing itself from starting because it has detected that the flap which controls the exit of exhaust gases is stuck in the closed position.

Flue non-return flap closed

What causes this fault?

This fault occurs when the boiler is unable to safely discharge the acidic water produced during the heating process. It is typically triggered because an external pump used to move this water has failed, or the internal electronic module that communicates with that pump has developed a fault. To prevent water from backing up and damaging the internal components, the boiler automatically shuts down as a safety precaution.