An ideal starting point would be to confront her, and inquire as to what is going on. What’s happening? What went wrong? Is she avoiding you because she is to afraid to speak up on some issue? Or is she just tired of the friendship and trying to quietly weasel out of it?
She is either trying to let the friendship die, or is working through a hard time in her life. You won’t know which it is until you ask – directly.
If she is experiencing some hard circumstances, find out why she did not reveal and then support her. Or, if she is just freely letting the friendship slide, then it’s time for you to put an abrupt end to it. Such a one sided friendship will only drain you, and be of no benefit now or in the future.