Look at pic #7 in 1g clutch , first link on page .http://vfaq.com/FAQlocator-drivetrain.html .

I would also pull out your speed sensor from the tranny, take out the g clip as I call them . Looks sorta like a c clip. Its what holds the shaft with the gear on it onto the speed sensor housing. I pulled mine out and found that the little plastic shaft that slides into the gear slot was broken. Bingo

Ignore the suggestion to look at the 1g clutch vfaq.

The speed sensor is bolted in the top of the transmission through the motor mount. A wiring harness connects to the top. I would bet that the harness is simply loose or disconnected. Of course if your are sure it is connected securely to the sensor then you may want to inspect its wiring or the sensor itself like by removing it and checking the gearing like bigbird said.

I would check the wiring connector at the speed sensor.