Thanks for writing. It sounds like something drastic and sudden happened to make your lilacs die that quickly. Do you fertilize them? Or do you fertilize lawn or other areas nearby? Too much fertilizer can cause the kind of damage you describe.
Herbicides can also cause that kind of sudden damage. Have you or any of your neighbors sprayed anything recently? Some studies have shown that herbicides can drift more than a half mile if you spray them on a windy day.
That's all that jumps to mind without being able to examine your plants. But if neither of those sound like they could be the case, you might check with the folks at your county extension office for more help. If you can bring them a sample of the plants, they should be able to help more.
---Justin, Senior Garden Editor, BHG.com"