well... number one call her... and just be honest how you feel about her. Like just tell her, that the time you really spent with her, has brought you to see her more as a good friend than a girlfriend. It's nothing personal, you just don't think it's best for the both of you that those feelings don't exist anymore. Then ask her to hang out and show her that it's not that you really don't like her.
I seriously hate when chicks text me about this kinda shit. I actually like talking to people when it comes to semi-important things and people's feelings.