Sounds like the solenoid on your gas valve may have failed.Check that the power is all on to the boiler(sounds obvious but there may be the odd hidden switch that's been turned off by mistake).Also could be the boiler thermostat.If you are capable and have the correct test gear you can check for power at the solenoid on the gas valve.
If it is the gas valve then a corgi reg engineer will need to change this for you.
Hope this helps.
Pete 100%Heating
Pete Brown, November 2006
