Your boiler has lost the signal from its room temperature sensor, so it can't read how warm your home is, but it will usually still provide heating and hot water.

DIY fix possible

Note the code 575 shown on the boiler display, then switch the boiler off at its on/off switch and wait about 10 seconds before switching it back on.Find the room temperature sensor or room control unit (usually a wall-mounted Vitotrol or thermostat in your main living area). Check that its batteries are fresh and that any wires or plugs connected to it are secure.Restart the boiler and check whether code 575 returns. If it does, contact a Gas Safe registered engineer, as the sensor or its cable will probably need replacing.