Does she ignore you when you're in public but talk to you when you're alone? If so, perhaps she's embarrassed and buys into the whole "popularity" stuff and doesn't want to ruin her reputation by being seen with a guy who's not in the popular crowd.
Or maybe she is just not that into you. Maybe she just chatted to you once and forgot about it.
Either way, it's fair enough for you to be a little insulted and rejected if you like her.
Maybe you could message her privately on facebook and just explain how you feel. Just briefly tell her that you felt interested in her and felt like she was too, but then felt like she suddenly wasn't. Ask her if she'd prefer if you just left her alone, no hard feelings or anything though. Then perhaps she will just say the truth.
There is a communication block... you won't find out how she feels unless you ask directly.
Just forget about her. Continue blocking her. If she can't forgive you for acting the way you did then she isn't the girl for you.