Your boiler was working but has suddenly turned off because the flame has gone out and it cannot manage to relight itself.

DIY fix possible

Check if other gas appliances, like a hob, are still working to ensure your gas supply hasn't been interrupted.If it is freezing outside, check the condensate pipe (the plastic pipe leading outside) for ice and defrost it with warm water.Press the reset button (a flame symbol with a cross through it) for five seconds to see if the boiler restarts.