Your boiler has detected that there is not enough water in the system, so it is limiting its performance to protect itself.

DIY fix possible

Locate the filling loop (a flexible metal or plastic pipe) underneath your boiler.Slowly open the valves on the filling loop to allow cold mains water to enter the system.Monitor the pressure gauge on the front of the boiler until the needle reaches the green zone (usually between 1.0 and 1.5 bar).Once the pressure is correct, ensure the filling loop valves are turned firmly off.