It sounds as though your tree has become infected with a fungal disease called ash anthracnose. This disease attacks white ash (Fraxinus americana) and green ash (F. pennsylvanica) just after bud break in spring. It is more severe in cool, wet weather. Symptoms often don't show up until several weeks later. The fungus causes large, irregular brown patches on young leaflets, usually following the veins. Leaves may be deformed, and leaf drop can be severe, especially in the lower part of the tree where the foliage is slow to dry.
By the time you notice leaf drop, the fungus has usually run its course, and no treatment is needed. It is a disease of early spring; as the weather warms and becomes drier, reinfection is unlikely. Even with heavy leaf drop, overall tree health is rarely affected; trees usually send out a second flush of leaves. If a tree has been infected, do what you can to promote growth and avoid stress to the tree. Water the tree during dry periods, fertilize if it's not growing vigorously, and mulch around its base.