Your boiler has detected that the water pressure inside the heating system is too high, so it has locked out to prevent damage.

DIY fix possible

Locate the filling loop valves attached to your boiler or pipework under the unit.Ensure both filling loop valves are fully closed and not letting extra water trickle in.Bleed one or two radiators in your home to release a little water and bring the pressure gauge down to between 1 and 1.5 bar, then reset the boiler.