I would maybe uncover the top of the tuber enough to see where the stem snapped - if it isn't directly at the base of the tuber use a little razor or knife to cut it about 1/32" to the tuber and you will get side shoots off of the base of the cut stem. :) Good luck!

Personally, I'd leave it be. It will re-sprout, no worries. There are probably more sprouts right under the surface that you might break when you go digging around.

As a person that has a hard time 'leaving it be,' I have broken a fair number of hidden sprouts being too nosy. <grin>

Lucky for us, dahlias tend to be resilient.

i feel like EVERY Dahlia I've ever head has had 2 sprouts? I've accidentally broken sprouts on Dahlia, Lillies, and Gladiolus) during planting, the required digging around to find out how the bulb is doing and when I will see the actually sprout (see cicivatation's post about digging around!), and once it's actually sprouted. Most of them have forgiven me and gone on to do just fine.

I have seen some of those tubers in a bag, late in the selling season. They actually had several stalks coming up out of the tubers. I just planted it like normal and they grew into healthy stems for the plant even though they had been growing in the bag without water or fertilizer.

Another shoot should be out by now!!! :)