Certified GKH Gardening Expert
If your lilac experienced a particularly harsh winter, it may be damaged leaves. However, it may have developed a disease. Inspect the leaves and stems for powdery mildew which is common in lilacs, and doesn't necessarily need treatment.
Here are a couple articles that may be helpful: https://www.gardeningknowhow.com/ornamental/shrubs/lilac/common-lilac-problems.htm https://www.gardeningknowhow.com/ornamental/shrubs/lilac/lilac-care-growing-and-planting-lilac-bush-plants.htm