If the goal is to be friends again, genuinely, then the first step seems to be to remind each other why the friendship is valuable. You can't do this with words usually... selfless actions or simple fun are both methods of re-conditioning each other into an environment where both look kindly at each other and begin to once again rely on one another.
... says Decius.
I'm actually a bit skeptical about that, since I've been trying that on a former friend of mine, who is semi-selfish (she only thinks about herself and one other person). I don't think it works on every friend, but most only.
Restless Mind, I would say look back at what caused this, and see if you can join or stop it. If not, then the best thing to do would be to, if you couldn't keep in touch, gather all of your memories with that person, keep it in one conspicuous place, and try to have fun with your other friends. Most likely, that will take your mind off the sadness and possible depression.