Thanks for writing. Like any plant, lilacs do have a lifespan, so it's possible yours is getting too old to bloom.
Or, if it's not in full sun, it may not be getting enough light to bloom.
Or, if it's all old branches, it may need to be renovated; cutting back a few of the oldest stems each year can help rejuvenate it:
---Justin, Senior Garden Editor, BHG.com