This could be so many things and if you are not experienced you should not try to fix yourself.
I would firstly check for the correct system pressure - usually 1/2 to 1 bar on the gauge, and refill if required. Secondly, I would check the high limit thermostat has not tripped out - this is resetable by pushin a buttton on a lot of boilers.
Please call an experienced engineer to fix this.
John, May 2008
