My friend had the same problem, but he chickened out. NEVER CHICKEN OUT! The girl will NOT like you then, trust me. They want a confident man. Anyways, if you like her, just say I like you too, and if you want to spend some time together. If you don't like her, then let her down easy, don't be all fast about it. Hope it helped!
One thing girls hate is to be ignored. Like you did. She probably would've been thinking what did I do wrong?? Dont chicken out. Tell her you like her if you do. If you dont then do what KC13 said at the end.