How to get a girl back and make it up to her?

Me and a girl I like, hung out one night downtown having a good time and she showed me that she was into me while we were together. I asked her out and the date went great, we both had a good time. We saw each other once after that and when I asked her on a 2nd dated, it was during her big test period and midterms, so I kinda backed off a bit and would call and ask how they are going. I asked her near the end of her hell week to our date night we had planned and she had a social to go to. (fraternity and sorority stuff) But I did not see her for a week or so, and I randomly bumped into her walking from cl and we talked, but it was not a good one, I could tell her was upset. So I called her up and left a message for her to cal me, because she was in a meeting when I called. She calls me back a day later and tells me she has been hanging with another guy, because she got attention from him, but her tone was kind of said like she wanted the attention from me. How do I do it?????