If your boiler is fairly new, it's probably a high efficiency condensing boiler. This involves running a plastic pipe out the wall to a drain, to carry away the condensed water, which in this severe weather is causing the pipe to freeze up, and the boiler stops working. If you can identify this pipe, and pour warm water along it outside, you can often free it and the boiler will work again, but often the boiler has "locked out" and you will see a flashing red light which needs resetting first..... check the user's manual. Had a really bad one today..... shite design, pipe about 5metres long across a flat roof, told the customer I could unfreeze it, but would be refrozen in 24 hours. In this case, if you're desparate, and you DEFINITELY know which is the condensate pipe, (normally white 22mm PVC), you could cut it with a junior hacksaw, and put a bucket underneath, doesn't need emptying very often, at least you'll have a warm Happy Christmas.