I have never seen my lilac bush do this before. It's a very old lilac and Bloom the color I've never seen before. I have lived at this home for 11 years and I've never seen the bush but this bad.
I think this could be a Syringa reticulata or Japanese Tree Lilac. This may explain why the leaves do not look right for S. vulgaris. S. reticulata does turn this washy yellow colour at the end of the year so it might not be much to worry about; it is sort of early for it to change but that will depend on local conditions.
Since there are no signs of dried fruit capsules the tree might be lacking vigour. Check back in the tree branches for signs of physical damage; I don't think they are subject to many pests and diseases but you might see something suspicious going on. Check the base of the tree for sprouts.