Every genuine cold snap produces the same call, several times a day: the boiler has stopped, it is showing a fault code, and it happened overnight. Very often the cause is a single pipe.
What the condensate pipe is
Modern condensing boilers produce water as a by-product, and it has to go somewhere. It leaves through a small plastic pipe called the condensate pipe, which often runs outside to a drain.
Outside is the problem. In a hard frost the water in that pipe can freeze, the condensate has nowhere to go, and the boiler does what it is designed to do: it shuts itself down rather than run in a condition it was told to refuse.
Why it always happens overnight
Because that is when it is coldest and when the boiler has been idle longest. People wake up to no heating and assume the boiler has died. Usually it has not.
What you can and cannot do
This is one of the few boiler faults where there is something a homeowner can reasonably do, and the manufacturer's instructions for your specific boiler are the thing to follow — they will tell you whether and how to thaw the pipe safely. Never use boiling water, and never use a naked flame near it.
What you should not do is keep resetting the boiler repeatedly and hope. If it will not restart, or if it keeps happening, that is a call.
The actual fix
The permanent answer is not thawing it every winter. It is dealing with the pipe: insulating it properly, or in some cases rerouting it so less of it sits outside. That is a small job in October and a miserable one in January.
If your boiler has stopped in cold weather before, it will do it again. Ask us to look at it before the cold arrives.