From the suddenness of the failure it may be that your oil level dipstick fell off the cap. That has been known to happen all too often. If that is the case your tranny is toast.

As Max said these Hurth/ZF transmissions are very expesive to repair.

Sorry but I see a new transmission in your future.

My original one had a leaky seal that, even when replaced and a speedy seal added continued to leak. Maybe the bearings were so worn that the seal could not handle the movement. Anyway, the cost to rebuild was 3/4 the cost of new so after 23 years of faithful service it went into the dumpster.

Shop around fot the replacement. I was in New Zealand when I needed one and the local dealer wanted over $5000 plus air freight from Austria. I found it in the U.S. for under $2000 and put it on a ship along with a whole lot of other refit materials for about $600.