You were the adult when they were a child. If their first instinct, as soon as they get out from under your thumb, is to completely ignore you forever, you need to own the fact that you messed up as a parent at several, consistent, points along the road throughout your child's upbringing. They hate you for a good reason, and they're probably better off without you in their lives.

There are a number of forms of abuse that range from over-parenting, to neglect, over-discipline to straight up negative enabling behavior.

I have friends who don't talk to their parents because the strictness was so suffocating, and friends who don't talk to their parents because they were lazy bums who never took an interest in their child's life. There are tons of other reasons kids abandon relationships with their folks, but the one thing that stays true through all of these experiences for me is that it's always the parents fault.

This is mostly about relationships that end as soon as the kid leaves the house, not necessarily relationships that break down during adulthood, although the same reasoning could be applied in a lot of these cases too.

EDIT:
To the people in the comments defending parents and blaming the kids: If you're a kid defending your parents then congratulations on having parents who treated you fairly and cared for you I'm sincerely happy for you (everyone didn't get the same life you did), if you're a parent defending yourself then just own up to the fact that you messed up and move on with your life, try to be happy and leave your kids alone because they've cut you out for very good reasons and they're better off without you.