Please help. What should I do if a girl I liked stopped talking to me?

I liked a certain girl (she was hispanic by tge way.) And we always hung out. I walked her to class around sixth period everyday and she used to hold hands with me and grab onto my backpack if I walked too fast. We flirted every day. But now she doesnt like me for some reason. I think it has to do with another guy,but hes not very nice, hes a user, and hes shallow. I think shes dating him now (she is always hugged up on him), but he treats her like ****. One day at 7th period, she was waiting by her locker for him. She called his name and he completely blew her off. My friends gave me advice, but I want to take the respectful gentlemanly way about solving this problem, but how is this possible to achieve if she ignores me now? Can someone help me out?