How do you repair a blender whose motor will no longer engage the blades?

I use a Black and Decker BL5000 blender three times a week on average, and after a few weeks of use, the motor no longer engages the blades. This has actually happened to me twice before with other models. I use the blender to make smoothies, which includes a chopped frozen banana.

Is there any way to repair my blender? I can't afford to replace them regularly.