There is, for lack of the correct term, a clutch plate or damper of sorts that attaches to the fly wheel. The transmission input shaft is splined and fits through the center hole of the plate, there are a series of springs that are in this part. The whole things acts as a sort of shock absorber between the engine and the transmission so it is not a clutching mechanism like you would find in a car. It is possible.......possible mind you, that the plate has come loose from the flywheel. The bolts have been known to loosen. I say this because it sounds like you have a physical disconnect between the diesel and the transmission. That would allow the engine to run but have no gear. If you pull the transmission and the problem is as I have described it will be very obvious.

Now, all that said, it could also be something within the transmission itself which I have had no past experiences with. Good luck.