The following could cause the sudden loss of drive between the engine and the wheels: #1 coupling between gear tran,(just ahead of gear tran) and reverse-o-torque unit came apart or splines on drive sheared in that area,  I can't remember if it was a chain type coupler, #2 reverse-o-torque ran low on fluid, has hydraulic forward &amp; reverse clutches similar to an automobile automatic tran's, has a dipstick to check fluid level, filter to keep the fluid clean. Other possibility, if you were changing from one gear to another on the gear tran's, may have had the lever jump out of a shift rail and gears still not engaged. When you run it the next time listen for a &quot;clunk&quot; or noise when you move the forward/reverse lever from one position to the other, if no noise it is either very low on fluid or has severe internal mechanical problem. Don't forget the manuals, they contain a world of info. Good luck, sorry I missed the S/N in your first post.