My first guess is that the freewheel lever is in the "disengaged" position. This is the lever located above and to the right of your trailer hitch on the rear of the tractor. It looks like a piece of metal clothes hanger, and can be pulled out to disengage the transmission so that you can push the tractor. Push it straight in, then see if your tractor will move. ALSO make sure the parking brake is released.

Note: If Javerts idea doesn't work, then check both rear wheels where they go onto the axle. sometimes one of the axle keys will work out, and then you get a no-go condition. RJ