If your boiler has stopped during a freeze, is showing a fault code and you can hear a gurgling or bubbling from it, the condensate pipe has almost certainly frozen. It is the plastic pipe, usually white or grey and about 22mm, running from the boiler out through the wall. Pour hand-warm water along it, concentrating on any bend or the open end, then reset the boiler. Use warm water, never boiling: boiling water can split the pipe and turn a twenty minute fix into a repair.