Ash Tree Yellowing
This looks consistent with Ash yellows disease. It is a phytoplasma infection that causes a witches broom growth of all of the foliage. There are a few insects that are vectors of the disease from one plant to another. There is no know cure or treatment for the disease and tree removal is recommended.
It can also be vectored to lilac, so planting other species of plants back into the area would be advisable.