How do I get my best friend to be more open?

Okay well my best friend used to talk to me a lot and say how much she appreciated our friendship and it was very liberating to hear that, it was great. Until one day when she got dumped by a boyfriend she really liked and she all of a sudden changed. It's hard to explain but I'll do my best. Of course she's afraid of getting hurt again so she stayed away from dating but also she did whatever she could to become stronger and less vulnerable. But by doing this she stopped being herself. She pushed away me and other people constantly to proof that she doesn't need people, she doesn't talk to me half as much (we sit next to each other in cl and we only talk half the time) and whenever we do talk she keeps the conversation very unpersonal and brief. Basically since she's prone to not being vulnerable at all the only emotions she shows are anger boredom and sometimes humor. I try stuff like saying I'm here for her but all I get is a simple "Thanks". And whenever I ask about something personally related, even something like a grade she got on a test, she only responds simply saying stuff like "nothing" and thats the end of it. I'm afraid to talk to her sometimes because a lot of the time I get the cold shoulder due to her being SO overly self relied. I used to be her one exception for somebody that she could be completely open with, how do I get it back to the way things were???