More than likely the drive chain, on the transmission internally, has broke and/or jammed up. For the wheels to suddenly just stop it's almost always going to be something internal on the transmission. This sort of thing rarely happens, but I've seen it before. There really isn't much to be done unless you feel handy enough to split the case on the transmission and fix the chain.
The problem with the tiller transmission is labor. It's not so much that it's hard work, but it just takes quite a bit of time. In general you'd be looking at 4-5hrs labor plus the cost of parts. Finding a shop to split the transmission open and fix it may be difficult. Even to replace the transmission you'll be looking at about 3-4 hrs of labor.
It's not hard work, just time consuming.
