Your boiler has detected that there is too much water pressure in the system, which can cause it to shut down for safety.

DIY fix possible

Check that the filling loop valves under the boiler are fully closedIdentify the radiator furthest from the boiler or a towel rail with a bleed valveUsing a radiator key, carefully bleed water (not just air) from the radiator into a container until the boiler pressure gauge drops into the green zone (typically 1.0 to 1.5 bar)