I just had to post an answer here because when I was 16ish I had a falling out with my father, at the time I vowed to myself never to speak to him again, I even sent him a letter telling him how the only thing I had learnt from him was how not to be a parent (teenagers ay?!!). Throughout the next couple of years he rang me a few times but I refused to speak to him. He stopped trying and after a while it just seemed like it had been too long to do anything about it. I missed him terribly at times but was I was convinced that he didn't care because he had stopped trying to speak to me, he never sent me cards on birthdays.
Anyway, I am 24 now and getting married to my wonderful bf which has got me thinking more of my father, when I questioned my sister as to whether she thought he might want to come to my wedding, she said as though she had been bottling it up for ages that he would love to and is so upset that he is not in my life.
We have still not made contact but my sister passed on my address and so I am waiting for a letter which of course I will be over the moon to recieve and am likely to call him straight away and arrange to meet him and for him to meet my fiance.
So the advice I would give is like the others have said, just send her a letter every now and again, a card on her birthdays and christmas maybe. Just to always let her know that you love her and you are thinking of her and don't give up she will come round in the end if you just let her know the door is always open for her...just don't give up!