Almost certainly the work you just had carried out has created debris which has made it's way down the pipe and has now collected in the narrow gap in the tap. You could either get the plumbers back to sort it or if you want to do it yourself you will need to isolate the water supply to the tap, remove the head and the top half of the tap, unscrew the innards, remove the debris and re assemble!
Since the plumbers caused the problem I think they ought to fix it for you, don't you? - for free!
Richard, January 2009
