Anger between people is a very common thing especially now a day. As people are very much conscious about their rights and demands towards life and whenever these demands and rights are not fulfilled by some of their friends or relatives, they get annoyed and angry. So far your question that your friend is angry with you because you could not attend her birthday party and now you are worried what to do, I can suggest you as follows.
First of all apologies your friend for not attending her birthday party and tell her the true reason of your absence. Then go to the market and buy a present of her choice and liking, go to her house, embrace her with open heart, kiss on her neck and cheeks and offer her present with best of your prayers and wishes. I am very much sure if you act so as I have advised you that your friend will forget her anger and welcome you with open heart. Even then if she shows her anger, take some help from her parents and relatives. I hope at last she will forget her anger and be a good friend of yours once again.
Friendship is a gift that is not given just once but day by day. This bond/relationship is a present from god to all of us from his precious.
Now the thing you have mentioned in your question is that your friend is angry with you that you didn't attend her birthday party, I think that she is right in her own view because as a friend of you she was expecting you to be there at her birthday party, but unfortunately you were not able to attend her birthday so she is angry with you.
As a friend you were supposed to be on her birthday. Anyway the thing you have to do is just visit her place with a present and wish her in a beautiful manner, spend time with her tell her your problem /reason that you were in certain situation that's why you didn't attend her party. And you will never do that again .I am sure that she will be fine.
But next time if you have any kind of problem or something just share your things with your friends never try to hide any thing from your best friends, because these thing sometime creates a real problem .So try to be natural in every way of your life.
Give your friend some time. It obviously was important to your friend to have you there. When things are less tense make time to talk about it. When we're not able to be there for our friends it can be seen sometimes as rejection. There may have been underlying issues too.... like not having a good day and really wanting to have you there to help make the day even better or other personal issues that can make facing the day difficult. I'm not sure that this is the case but it is important to make sure this gets resolved. Keep open and available to your friend. It may not even be necessary to bring up the birthday party. Only you can make that decision. But I think you'll know.
Friends can be the most precious asset in anyone's life. A good friend is the blessing of God. A true friend can not be angry for a long time.
Your friend is hurt but she will be normal within a few days. You should go to her home and tell her the reason why you cannot come to attend her party.
Friends play an important role in a person's life. Friends give you relief when you cry and when you laugh they share your joy. They give you comfort in pain and share your problems. It is very hard to find a sincere friend but if you found then don't lose them at any cost.
Some times friends got hurt by our doings but it doesn't mean they don't love us.
You should call her and go to her home and try to convince her how much you love her. But some times there are lots of any other problems. I hope she would not mind and forgive you.
Friend is the most precious gift given to the mankind by God. Friendship is a very beautiful relation and must be dealt in a very loving manner. Beside this loveliness it is very strong relation and small things never affect this powerful relation.
If your friend is angry at you as you could not attend her birthday party then do let her know the proper reason behind that act. If you deliberately miss the party then you should apologize because you must not have done this. And if there is a certain proper reason then do let her know because she might be a very understanding friend. In fact talk to her and give her some sort of beautiful gift she likes. She might have missed you on her party and that might have hurt her a lot, that is why she got angry with you.
Always take care of your friends and friendships because a true and sincere friend is the blessing and so is the friendship. Try to make a friendship bond stronger by talking to her about the reason of missing the party and do present her a beautiful gift or a bouquet. I am sure your friend will understand and you will be having your best friend back with a beautiful smile on her lips and at her heart as well.