Likely the rotor in the speed sensor mounted on the right side of the front wheel has broken. It is a common problem with the 03-06 400.
It's easy to find out if that is the case. Just pull the front wheel off to gain access to the sensor. Gently pry the seal out of the sensor being careful not to bend or damage it. Pull the rotor out and see if it is still in one piece.
Many have been able to fix it by just taking the rotor out and gluing it back together. I did that to mine at about 7,000 miles. It lasted until about 36,000 miles when it broke again. The second time it was damaged to much to repair again so I just ordered a new rotor.