Make sure the cord is secure into the power plant.  The newer models, G-series, have a screw holding the cord stable, but over time they can still work loose. 

Take the Nozzle off and reattach, making sure nothing is interfering with the speed selector safety switch.

Finally, make sure the bag is secure,  remove the emptor and reattach, again making sure the switch is making contact.

If these steps fail, check the cord, feeling the entire length to see if there are any broken wires.  

That's all I got, others may also suggest the brushes and such I would start simple and work up.

One last thing, have you tried other plugs, or made sure the plug you are using is live.  Sometimes the switched plugs get switched off, or the breaker has flipped.