Being a female, here's my take:
She was clearly interested in you and gave you many signals that she was. After a while when you never gave her any indication that you felt the same way, she probably just decided to move on as she felt there was no chance of being with you.
Honestly, if you just admitted to basically ignorning her, are you that surpirsed that she no longer shows the same interest in you? I'm afraid you may have missed your window of oppurtunity with this one.
However, maybe just try talking to her once and get a conversation going. Show her that you do have an interst in her, and maybe she'll show an interest in you again. But don't just ignore her anymore, that won't get you anywhere.
*edit* lurekette posted while I was writing, and I just wanted to say ditto to everything she said. *end edit*