Make sure that the bolt that runs from front to rear on top of the gearbox is tight. This bolt holds together the gear selector fork which is in two parts. One part is the external gear selector quadrant and the other is the internal fork assembly. The two parts have a male “V” and a female “V” are held together by the through bolt. If the bolt is loose, the two parts disengage and you can’t select a gear or neutral.
Dave
