The transducer is powered from the reverse light circuit. Check if your reverse lights are working. If they aren't then check the fuse (usually if you lose power to the transducer, the speedo will get to approx 80 km/h then drop to zero, then back to 80, and back to zero repeatedly). If you think the problem is in the gearbox (unlikely on a vn), unscrew the the sensor, plug it back into the wiring loom and spin it up with a drill. If still no go, better take it to an instrument tech. Where are you situated? Could probably suggest someone to take it to.