I feel so guilty and I don't know what to do about it.
Oh no, what do you feel so guilty about?
I think I did something to upset my best friend.
Sorry you are feeling bad. It's good that you are so concerned about your friend. Try thinking about what might be making you feel this way and then apologize to your friend if needed.
I've tried, she's ignoring my calls. I don't know what to do now.
If she won't answer your calls, try sending her a card or a present and include a written note asking if the two of you can talk. I'm sure you can work past your issues and repair your friendship.