The boiler has lost its connection to the wired thermostat on the wall, so it may not be controlling the heating as set, though it can often still run on its default settings.

DIY fix possible

Turn off the boiler at its on/off switch and at the fused spur or isolator, then wait two minutes.Check the thermostat cable at the wall plate and at the boiler end for loose, damaged or disconnected wires, and reconnect any that are loose.Turn the power back on and wait around five minutes for the boiler and thermostat to re-establish communication.If the code returns or the heating still does not respond to the thermostat, call a Gas Safe registered engineer.