Best friend break-ups are the worst, everyone knows that nothing last forever. It's not the easiest thing to get over.
Step 1:If this friend wants nothing to do with you, for whatever reason, there is only one thing you need to do and that is to accept it.
Step 2: Accept the fact that the person might have changed
Step 3: If you lost them because you told them how you feel such as, about them, or an issue they ask your opinion on and they get mad and don't want to forgive you, don't feel bad. It's their lost, most likely they were never really your friend.
Step 4: Don't be petty. Don't tell everyone or put up on social networks what your former friend shared with you (secrets).
Step 5: Let go. If your former friend doesn't forgive you try and move on and know that you will find other friends. Also take a break and get ready to start new.