it would be nice to know
boiler make & model
gas council number
any fault indicator lights flashing or fault codes?
there is no way of narrowing list of possibilities that could be causing fault at the moment with information available 
in the absence of actually standing in front of boiler & conducting a series of simple tests im afraid i have to rely on yourself for more info in order to help
                
 manwithnoname
                
 April 2012