Check the fuse in the power plug. If you don't have a tester, replace with a fuse from another similar tool and see if your tool works. If not, then look for the brushes on the motor from the outside of the tool. If you can locate them, determine if either one of the two brushes has lost contact with the motor. If you find one of the brushes, look on the opposite side of the motor for the other one. They are always located opposite one another on the motor.
Open the case of the motor and remove the brush caps. They are usually very brittle so be careful. Examine the brushes. Determine if they are still long enough to touch the armature; if they are then you probably need more than brushes. If one brush is worn more than the other one and doesn't reach the armature, you probably only need to replace the brushes.
Remove the brushes from their mounts and order replacements. Be careful to order the correct replacement brushes for your tool as they conform to the geometry of the individual motor they fit. Install the replacements when they arrive in the reverse order of removal.