What would you do if you found out your dad changed his name and went no-contact with his family years ago, before you were even born?

Hypothetically, you find out your dad changed his last name because he hated his father. Let’s say he changed it to the last name of a fictional character he really liked, then went completely no-contact with his family decades ago, before you were even born. Then, 20-something years later, you discover your grandmother is still alive, and your last name isn’t actually your dad’s birth last name.

What would you do?