You tell her the age appropriate truth as she is able to hear it.
1. Of course you have a father, but he decided he didn't know how to show you love and care for you the way you needed to be so he lets all of us love you. AND then list all the stable wonderful people in her life. You, Grandma, Grandpa, Aunts, Uncles, Cousins, family friends, mentors coaches teachers...etc
2. Your father and I met and made you at a very young and unstable part of our lives and we decided it was best if you stayed and were cared for by me and lists of people who are stable and loving in her life.
3. You father's name is ____________ I dont' know where he is but he made the choice to give you the best and most stable life posible b/c he couldn't promise to maintain that for you. So he decided that I raise you with LISTS OF PEOPLE who are stable and loving.
Stay positive, matter of fact and that it was something wrong with him that he couldn't take care of her and made a loving decision not to distrupt her life and that nothing is wrong with her he just isn't capable of showing her the love and care she needs.