no hot water?

You provide precious little information - heating systems are not all the same...

They sometimes have a valve to send the heated water to hot-water tank - radiators or both.
Yours may be stuck...

This video will explain it a bit...

https://www.youtube.com/watch?v=Yyn_fBEH90U

If you have a hot water tank with an immersion heater
then Bob's your Uncle!

You will be lucky to get hold of a plumber before the Christmas holiday.

Good luck...

Peccavi
December 2012