You do not have to prune lilacs. You can drive through the Midwest and see abandoned farms, nothing but the foundation of the house left, and lilacs that are over 75 years old in full bloom. I love to think of the people that planted and enjoyed them.
If you want to prune them, yes, you have to prune them the right way, like most things.
I decided not to prune mine and they get more and more blossoms every year. If they get leggy, maybe I'll prune them.
I found this. You can call your local university extension and see if a master gardener will talk to you. I think it's likely your lilacs will bloom again, though.
https://hortnews.extension.iastate.e...993/lilac.html