Sorry to hear about your tree. We've sadly had a bad situation in our area with something the OSU extension office identified as Hypoxylon canker in oak trees. The fungus affects mature trees, mainly, and will kill a 3-4 foot, trunk diameter, tree in just a few days. It causes the bark to pop off the tree. Very odd. Apparently most trees have this fungus but it attacks and kills stressed trees. Well, the summers of 2012 & 2013 for us sucked for rain so I guess we had a lot of stressed trees. The OSU folks told us to keep the trees stress free by watering and feeding. Best of luck.