The boiler has lost contact with its room temperature sensor, so it can't properly judge how warm your home is, but it will usually keep heating and hot water working in a basic way until this is checked.

DIY fix possible

Turn off the heating controls and switch the boiler off at its on/off switch, then wait 5 minutes.Check the room sensor (the separate wall-mounted or thermostat unit in your living area) is securely attached, and check that any visible wire leading to it is not loose, cut, or damaged. Do not open the boiler casing.Switch the boiler back on and reset it using the reset button or by pressing the reset option on the display, then see whether the F.57 code clears.If the code returns, stop and contact a Gas Safe registered engineer to test the sensor and its wiring at the boiler.