Your boiler has lost some water pressure, so it is working less efficiently to prevent internal damage until you add more water to the system.

DIY fix possible

Locate the external filling loop, usually a silver braided hose with one or two valves underneath your boiler.Open the valves slowly to allow mains water into the heating system while watching the pressure gauge on the front of the boiler.Stop the flow once the needle reaches the green zone, typically between 1.2 and 1.5 bar, and ensure both valves are firmly closed.