Your boiler has detected that the water pressure inside the system is far too high for it to operate safely.

DIY fix possible

Locate the filling loop under the boiler, which is usually a small silver braided hose with one or two valves.Ensure both ends of the filling loop are firmly closed, as a leaking valve is the most common cause of rising pressure.If the pressure remains high, vent (bleed) one of your radiators slightly to release excess water until the boiler pressure gauge drops to the green zone (typically 1.0 to 1.5 bar).