Could very well be Verticillium wilt, a soil-borne pathogen that is omnipresent in most soils. It can take down a susceptible tree or shrub overnight. Lilacs are susceptible. And you cannot replant in the same spot with another susceptible species.

If you had a frost just after the leaves opened, there is a good chance it is a bacterial disease called Pseudomonas syringae pv. syringae. Not much you can do except cut out the dead branches, trim the plant back and give it some fertilizer. It should not spread to the healthy portion of the plant.

Lilac blight - Pseudomonas syringae - can occur at any time during the growing season, not just as a result of frost damage, although it often will manifest after a cool, wet spring. And it is very contagious, spreading readily with wind, rain and irrigation. It is one of the most widespread and common plant pathogens.

my lilac just started blooming.. and are barely leafed out... its been so cold.. specially at night ...that it would be more likely to be a cold issue.. rather than a disease process .... when night temps are still possible in the 30s.... unlike the pnw... its the same reasoning of why there arent mosquito issues when night temps are so low ....

dont get me wrong.. all the diseases are possible..

ken