one person has to be brave enough to admit that (s)he was wrong and apologize. then ask for forgiveness. be friends again. it won't always be the same after that though. The friendship should become stronger after that if both person have forgiven each other.

Apologize directly and if it would be awkward to acknowledge the mistake then make up for it indirectly in a way that makes the other understand,that works very good in relationships like a small gesture or a present ;)

I agree with Niney Chan and just add: if you see you can do anything to mend or improve the situation, just do it!

Reconcile with apology.

send a nuke to his/her location. kidding. --- talk it out with him/her. or with a relationship authority who can ease tension between you and him/her. send a card, a note. flowers. anything that's not crappy after what seemed to be a global feud. buy him/her food and drink and have a nice time with one another. or just ignore what comes one's way that can aggravate a situation. sleep and mention casually that the person you care about but has hurt feeelings is a special case of human being on the planet on the way to world peace and personal happiness for everybody else who're not quarrelling. be happy. have a nice time patching the patches to more happy friendships.