It sounds like your tree is having problems providing enough water to the canopy to keep the foliage and cambium alive. If you are not watering it, that may be the problem. If you are watering it, your tree may have contracted a root disease. In that case, it is important to clear all organic material at least 3 feet around the stem, slightly expose the root system next to the stump to the air, avoid fertilizers, and water minimally.
My pear tree was looking great. Within a week one of the limbs died and now it looks like the whole tree is dying.