Need Advice

I am seeking advice because I am unsure what to do concerning a daughter that I put up for adoption a little over 13 years ago. I was a teenager and so was the baby's birth mother. We actually went on to get married many years later, have successful careers, money, and children of our own. I realize this is not the most traditional adoption story. Through the wonder of the internet and some investigation, I have been able to find out information about my birth daughter's adoptive parents. They moved to a new state when she was a baby and her father died when she was very young. I believe he died of a drug overdose. I am also aware of the mother's address but have no idea of the health or well-being of my birth daughter. My question is whether or not it would be appropriate to make contact with the mother through an initial letter. I would like to at least let it be known that this could be an open door for answers or perhaps a potential relationship with both her birth mother and father. My concern is whether or not this may be harmful to the young girl (13 going on 14) or my own children (who are unaware). This has been a difficult trauma to put behind me and I don't know if I ever really have. Does anybody have any advice or suggestions about what may be the appropriate course of action?