It could be either the brushes as Paul said or it could be the switch or your cord. Depending on the model of your drill the parts pricing can vary. If you have a model # of the drill I can give you a better idea.
Don- www.toolsandmachinery.com
February 2009
very probably the brushes have gone. Did it act slightly irratic before ceased to work at all?
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix drills.