Hi Beverly,
Thank you for your lilac damage question. I cannot definitively determine what is happening with your plant from your photos, however, it could be lilac blight. The following article describes this plant disease. Photos and management options are included:
How to recognize, treat and avoid lilac bacterial blight | OSU Extension Service (oregonstate.edu)

To keep your lilacs looking their best, they need to be pruned, fertilized and shaped almost every year, soon after they are done blooming in the late spring. Don’t wait too long, as next year’s blooms start forming in the summer. You don’t want to cut them off by mistake. Pruning in the winter will remove all the bloom for the following spring.
Source: Prune lilacs soon after bloom | OSU Extension Service (oregonstate.edu)

If you have more questions or need additional information, I recommend you contact the Plant Clinic at the OSU Deschutes County Extension in Redmond. You can call the phone number listed below or send your photos and question to <personal data hidden>

The person on duty will review your information, conduct necessary research, and contact you with findings or management options.

Summer Hours: Monday through Thursday, 9:00 to 5:00 pm, Friday 9:00 am to 1:00 pm.

Phone:<personal data hidden> x79630

We always welcome your home garden questions at the OSU Plant Clinic.