She was friendly now she cold, what should I do?

Ok so there this girl at school she used to be all into me but now she just completely ignores me and I’ve noticed that she will talks to everyone else throughout class but act like I don’t exist at all, she was even seating next to me and now she changed place, she was hella nice and flirty to me b4 that shit hurts so much I love this girl so bad I don’t want anyone else except her What did I did wrong? How can I get her back?
She was friendly now she cold, what should I do?
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