I must tell you a sad story from my own experience. A number of years ago the same thing happened to one of my apricots too and exactly as you describe. It was when I was a beginner gardener. Only when I lost an apple tree did I seek out advice. Here is something to think about. If you are watering by hose and it is laying out, the water can get blasted hot. I burned the root ball on my trees by simply turning the water on in the summertime as the hose lies at the foot of the tree. Some trees are really sensitive to that. If it were a mesquite I would say probably no problem. Let the water run outside the foot of the tree until it is cool to the touch. Then plop it in the well. I am suspicious this is the problem since it happened quickly. If the wood is green still, I would try to baby it back to life. If it snaps off and is dry inside, it might be firewood. I water 2 - 3 times a week in the summer. Summer is tough on the fruit trees; the leaves will fold when they are thirsty.