Thank you for the question. Ash trees can suffer from disease, insect, or cultural and environmental problems. Without seeing the tree, we can't say what the problem is but you might be able to figure it out by looking at our self diagnostic website, "What's Wrong with my Plant". Here's a link to the section on ash tree problems: http://www.extension.umn.edu/garden/diagnose/plant/deciduous/ash/
If you can't figure it out from this, take some good photos of the entire tree and close-ups of the problem if you can and re-submit your question.
Good luck and thank you for contacting Extension.