Typically when relationships end they end with thoughts that there is something better around the corner. The fact is, there very well could be something better around the corner but that something better could have a whole host of problems. It sounds to me as though you two split for a reason and then when the going got tough you found yourselves back together.
My guess, she either regrets her decision and is unsure how to let you down easy or she is really going through some **** and you're sitting around bitching at her for not being able to see her. If you really do love or even like her a whole lot then you'll knock off the needy boyfriend crap and give her space. The best you can do is remind her on occasion that you are there for her. While she may not forget it, it's nice to remind people from time to time. I don't mean every other day, but every so often. If the two of you are meant to remain together then you will.
Since you didn't state your ages I'm also guessing these are typical growing pains. She clearly needs space and you're not giving it to her. You could dump her today, which is my solution, but you could be doing more damage. What do you have to ask is what is she taking and giving in the relationship? Problems or not if you know nothing about what is going on then you are well within your right to leave her. If she can give you an idea of what is going on you can decide whether you wish to wait it out for her.
These are decisions you have to make but you have to man up and be mature about this situation. You have to tell her (gently) that you'd like to know what is up and remind her that if details are withheld then that is ok. Tell her you want to better understand if this is temporary, having to do with you, or even something that you could help with. Once you know you can decide what your next steps are but if she is unhappy and refuses to see you then the answer to your question about how to make her happy is a moot point. If you cannot see her and she won't talk to you then the way to please her is give her space and time. However, as with any relationship; romantic or not, she at least should give you some idea as to what is happening with her. As a decent human being she should at least do that.
Good luck to you but in the end I think you may be young and I think it may be time to move on. If you want, stick it out and resent the **** out of her. That is always fun.