Based on your description, your Wisteria was likely hit by Canker (Botryosphaeria) or Coral Spot, common fungal pathogens that enter through open wounds. Even though you used Japanese putty, any moisture or spores trapped underneath during winter would have created a perfect breeding ground; the fungus then attacked the branch's vascular system, blocking the water flow so that the whole branch collapsed when it tried to bloom in spring. To save the rest of the tree, prune the dead branch completely back until you see 100% clean, green wood (making sure to sanitize your tools between cuts so you don't spread it), and apply a liquid copper fungicide to dry out the wound instead of a heavy paste.