Hi Sydney,

Many lilacs are getting bacterial blight this year. This usually doesn't kill the plants. Some even sprout new leaves after dropping all the brown ones.

Here's some helpful web pages:

https://yardandgarden.extension.iastate.edu/encyclopedia/lilac-bacterial-blight

http://plantclinic.cornell.edu/factsheets/lilacbacterialblight.pdf

If you don't think this is blight, this page may help you narrow the cause:

https://apps.extension.umn.edu/garden/diagnose/plant/deciduous/lilac/

Both of the links above suggest ways to manage blight, including raking up and disposing of fallen leaves and, when it is dry, pruning to open up the plants and allow better air circulation.

That’s all you can do right now. Good luck.