Your boiler has been manually set to a testing mode used by engineers to check its performance, which overrides your normal thermostat settings.

DIY fix possible

Locate the chimney sweep switch or button on the control panel, which usually has a small icon of a chimney or a magnifying glass.Press and hold the button for at least five seconds until the symbol disappears from the display.The boiler should return to its normal heating and hot water mode automatically.