Most Helpful Girl
She may have gotten the idea that you like her but doesn't feel the same way. To avoid that awkward situation of either accepting or rejecting you, she's being nice by not ignoring you but also not leading you on.
I've been that girl a few times so my answer is based on my experience.
Most Helpful Guy
She might've stopped liking you or found another guy or something.
I was really good friends with a girl in my high school for junior and senior year (probably my best female friend throughout high school), and I didn't see her since graduation (last spring) until a few weeks ago when I saw her for like 3 seconds at my job, but she was leaving the store, and then I texted her that we should hang out, and no response since then.