Would it hurt to duct-tape it back together? I doubt it would grow back, but I can't see how it would hurt, other than accidentally snapping the other half too. That's a big pumpkin!!
Sounds hokey, but I was able to save zucchini plants from vine borers by slicing the stems open. Sometimes the unconventional can really do the trick. You may ask what kind of nut tries to save zucchini plants, but I usually only plant 2. :-D