Ah Christmas - Wonderful time of year... You are not the only one... Had 4 customers this morning and 3 more still waiting for me...!
Check all the controls are calling for heat, is there power going to the gas valve? Try extinguishing the pilot and relighting it; ensure the overheat thermostat has been reset.
If this is one of the fanfare apollo's it could be your fan stuck or aps, see if the fan is running by placing your hand on the flue outside and feeling for air movement.
GasAid.co.uk, December 2006