Your boiler has automatically turned the heating off because the weather outside is warm enough that it thinks you don't need it.

DIY fix possible

Check your thermostat settings to see if you want a higher indoor temperature.Adjust the 'Summer Economy' or 'Heating Limit' threshold in the boiler settings if you want the heating to stay on during warmer days.Verify that the outside temperature sensor is not in direct sunlight, which may be giving a false high reading.