Your boiler has lost enough internal water pressure that it cannot operate at full capacity, so you need to top it up to restore normal heating and hot water.

DIY fix possible

Locate the filling loop (usually two silver valves connected by a braided metal hose) underneath the boiler.Open the valves slowly to allow water into the system until the pressure gauge reads between 1.0 and 1.5 bar.Close both valves tightly to prevent over-pressurisation and check for any minor leaks.