Dear Debbie,
You might need to do more than simply say "I'm sorry." Tell your friend that you feel bad about the fight and care about your friendship. Let her know that you understand why she's angry, and ask her what you can do to make up. Your friend might be too angry to really hear your apology now, so give or send her a note expressing your thoughts.

Dear Debbie,
Your friend is probably still upset about the fight. It is good that you stepped up and tried to apologize, but she just wasn't ready. Give her some space to cool down and soon enough she will be ready to hear your apology.