Could be the brushes burned out. Don't panic, they are user replaceable. If you have the user guide it should explain exactly how to do it. If you don't have the guide you'll first need to get a set of replacment brushes from a larger hardware store. Make sure to get brushes for your model. On the side of the tool are black slotted caps that can be unscrewed BE CAREFUL THE SPRINGS UNDER THE SCREW CAP CAN GO FLYING. With the cap removed the old brush will fall out and can be replaced with the new one. 2 brushes, one on each side are needed. The 400XPR is a newer model so if this doen't repair it, there may be a defect in the tool.