a carbon brush is a block of carbon on a spring that contacts the armature to provide power
https://shop.ukwhitegoods.co.uk/hv12101-washing-machine-carbon-brushes
this block wears down over time
you can see the picture the length of the block. you would need to remove the brushes and check if they have worn or jammed for some reason
or you could ring the engineer and tell him it may have broken under guarentee. if he is honest he will come back and check and only charge you if its a different fault
Dave