I agree about the "just fell off" statement as the rather deep scratches in the paint say otherwise.

Your first and best option is to talk to Co-Motion but if they can't help or it's too expensive, here is one "fix" that is permanant but requires a bit of work and faith in your abilities.

1. Grind off the remaining broken stub and the one on the other side of the downtube.
2. Drill a 5 mm (or 3/16") hole straight through the current stop base and all the way across through the downtube and out the opposite side.
3. Get a pair of Shimano downtube cable stops. They come with every STI or barend shifter set so most bike shops have a bunch of surplus sets.
4. Buy a 4" #10x32 bolt and a nut.
5. Run the #10 bolt through one of the cable stops, through the downtube holes and out through the other cable guide. Add the nut and tighten snugly. A drop of Loctite on the threads is highly recommended.
6. Cut off any excess bolt and file the end flush and smooth.

If the cable stops want to rotate on the bolt, bedding them in epoxy before installing the bolt will prevent it.

I did this fix on a friend's older Klein and it has worked perfectly for years.