The brushes remain staitonary and deliver electrical power to the rotating part of the motor. It's unusual for them to suddenly stop working - you often get poor motor performance before they fail completely.
You will find them on the motor - often inside little, clip-in, plastic carriers.
You may have a defective motor or motor control electronics but brushes are relatively cheap and relatively easy to replace.
This vid shows how...
http://www.youtube.com/watch?v=0GmtRBiXLFc&feature=PlayList&p=F82654651FCE7539&index=0&playnext=1
Good luck...
Peccavi
June 2009