Frozen condensate pipe. Modern domestic gas boilers have to be of the condensing type (if yours is less than a decade old it will be)- in short wasting less energy because the flue (exhaust) gases have much of their heat removed, and the water vapour therefore condenses. This is just like in the bathroom where hot wet air condenses on a cold window. That condensate is funnelled through the boiler and out in a plastic pipe.

If this pipe goes outside, and the temperature drops significantly (like 2018's "Beast from The East") the 'condensate pipe' can get blocked with ice, and the boiler will eventually stop working... (Some boilers show a fault code, but some just drown) . Pour a kettle - or 5  ! - of very hot water (slowly) onto the top of the outside part of the plastic pipe (condensate pipes must be plastic). BE CAREFUL - you are holding boiling water and the ground will be icy !!!! 

It may well clear (that's what we do initially and it's 90% effective) . If not, call Sevenoaks Gas and some other issues may be present.

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No circulation.  If you have checked the pressure and still no heating or hot water, your pump may have failed.  More rarely the system is so gunged up (with magnetite) that the water cannot circulate. You need an engineer and possibly a power flush (if the pump cannot move the fluid around). If you regularly need a replacement pump, you might not have a bypass valve - required in a pressurised S-Plan system.

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Rats ! I recently saw a boiler that had been chewed by a rodent (probably a rat). They are known to chew electric cables (rarely, causing house fires) . In my recent case, after an appetiser of two electric cables, the main course was chewing through a thin plastic water pipe! The result was water showering the inside of the boiler and blowing the pump's PCB and windings and the main PCB fuse.

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If you live in the country, or especially if your boiler is in an outhouse or lean to, get an ultrasonic rodent repeller. Various stores sell them ...

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