Lilac leaves turning brown
We can't diagnose the problem by viewing the photos.
However, verticillium wilt sometimes causes the sudden death of trees and shrubs. Go here to learn about this disease:
For a certain diagnosis, send samples of dying branches to the University of Minnesota Plant Disease Clinic. Verticillium is soil borne so if it’s present, it’s not advisable to replace the lilac with other verticillium sensitive plants.
Go here to learn how to prepare samples for testing: