How to find a person...

Okay so when there is no adoption process or relinquish of parental right . ie:the parent just walked out. How do you find a person?

Example one of the parents walks out and never comes back when the child is a baby and the said child is a adult quite a bit over 18. My children has started asking me and my husbands questions and ofcourse we have no answers and not a lot to go on. I do have a name and very very and I mean very limited information on said person of interest. Not sure where or how to even start to get answers any ideas?