How to win back a sweet friend you've lost?

I hate to say it but it seems like you kinda missed your boat as far as a relationship with her. She is probably put off by you because she was thinking you would ask her out but instead you chickened out! I would be very straight forward and have a one on one conversation about what went wrong because if there is any hope for you it's that she will communicate to you why she is avoiding you. And if you can do that there may be hope in resolving the issue.