Many adoptions are closed, meaning the birth parents aren't revealed to the adopted child, and consequently the child has no interaction with his or her birth parents. There are many reasons why an adoption may be closed. More frequently, however, both adopted parents and adopted children are seeking to open closed adoptions.
"By whatever manner families arrived at closed adoptions, many are showing an interest in changing that, and establishing contact among all three parties - birth parent(s), adoptee, and adoptive parent(s)..."
Adopting.org has posted a chat discussion between a birth and adoptive parent who decided to open their adoption. It gives great insight into the reasons behind, and process of, opening a previously closed adoption.