Your high temperatures made international news. Compounded with your current drought conditions, it's very possible that your lilac is stressed and in survival mode. Given your usual weather, you're probably not accustomed to watering your trees, so a long, slow soak could help it through the rest of the summer. However, it's important to rule out disease or pests as well. Are you aware of other lilacs in your area suffering the same symptoms? Have you examined the leaves and branches for signs of pests or disease? A local nursery ot garden center might also know of any regional lilac problems.
It will help if you can post close-up photos of leaves, both dead and alive (if any), bark and branch tips.
Here is an article about common lilac issues in the US
https://extension.umn.edu/yard...
One of our members who is knowledgeable about trees and lives closer to you might be able to assist
@Silversurfer
