I'm upset with my long distance girlfriend for cutting contact with me for a month.
Why did she cut contact with you for a month?
I have no idea. She just stopped talking.
It's okay to be upset when a significant other stops making contact with you, especially for a month.  See if you can get her to tell you what's wrong, but if she refuses to respond, you may be better off just moving on.
So what are you saying? I have to give up on her?
I'm not saying that you have to give up on her, just that you have a right to be annoyed about the situation. If you think that giving up on her is right, then that's your decision.