Your boiler has switched itself off because the water pressure is too low or the pump that moves water around your radiators isn't working correctly.

Fault on Pump or Low System Pressure

What causes this fault?

This fault occurs when the boiler detects that there isn't enough water pressure to safely operate or that water isn't being pushed through the internal heat exchanger. The most common reason is a simple drop in system pressure, but it can also be triggered if the pump has seized or failed due to age. If the water cannot move, the boiler shuts down to prevent the internal components from overheating.