Analyze the case, which was perceived your friend as a betrayal on your part. Certainly, justifying himself, you talked about the fact that he did it accidentally and not on purpose. Don't lie to yourself, and if it were not so, and your action was dictated by jealousy, greed or a desire to hurt, think about whether or not to return a lost friendship. If you are experiencing similar feelings to the person who trusts you, then there is no guarantee that this will not happen again. Apologize and go their own way.
In that case, if you, having realized its error, sincerely repent and want to regain the trust of a friend, the only option would be an honest conversation in which you truthfully tell us about the motives that you are forced to do so. If she stopped communicating with you then send her an e-mail. Try not to justify itself, nor the circumstances, nor a bad mood, or other reasons. Give a promise not to repeat such mistakes and ask her to forgive you.
Be prepared for the fact that if you forgive and return the friendship, the relationship, especially at first, will not continue and will reach a new level. Behave with a friend exactly, does not show a constant feeling of guilt, not engage in self-flagellation, because you can go back and fix done or said you can not. Just try to be honest and cherish your friendship, which had to go through the test. Work on yourself and overcome those traits that can hinder your friendship.
Life goes on, and there always will be situations which will serve as a test of your friendship. If you together with honor out of these tests, and make sure that this feeling for you is dear and Holy, then your mistake will eventually be forgotten. If your friend will forgive you, then your friendship will have the opportunity to become even stronger.