No "neutral" position per say. Neutral is selected by the particular position of the rod on the HST, once it is moved away from that position, either forwards or backwards the machine will move.
No forward or backwards movement with HST rod travel? First check would be hydro oil level, and type. Second check would be that the engine output is actually connected to the HST input shaft, is there a broken driveshaft, failed V belt, etc?
Please post what you find
David