Test the cord with a volt-ohm meter (aka multitester). Disconnect the cord from the machine, and with the meter on the ohms setting (marked by the Greek letter Omega), put one probe into one side on the female end and touch the other to each of the male ends. The meter should react (full needle deflection on an analog meter or some sort of number on a digital one) when touching one of the male ends. If it reacts to both, you've got an internal short. Repeat the process with the probe in other female end and you should get a reaction out of the opposite side that you got before. If not, the cord is bad.
Also, you might want to--with the machine unplugged, of course--take the nozzle off of the front and try to turn the fan with your finger. This will tell you if the armature is frozen up.
As others have noted, Kirby motors are pretty robust. They don't just quit like those on cheapo plastivacs made of chineseum.
