When is a friendship officially over?

I look back on all of my long-term friendships and wonder what exactly we did as friends to keep them alive even after years apart. Is it just a deep abiding love for each other? Is it mutual respect, a sense of calmness? I have people in my life I grew up with and I can be apart from them for years as I lived away and then see them and it is like there has been no distance, either miles or time.
Then there are friends that I've worked hard to keep the friendship going until a time when I realized I was the only one putting in any effort. So I put out as much effort as they, and the friendship seemed to wither; not a word of explanation.
I asked one such person a couple of weeks ago on the phone "is this over?" I meant is our friendship and association, in his eyes, finished? I asked this because I was receiving cold responses, and an complete lack of feeling.
In that case, I left him alone. His silence said it all. Silence says, "I no longer want you in my life." No other explanation, or response. I understand.
But friendship is a very valuable commodity. When you become my friend it means I make a commitment to you. I'm there to help, to support, to lend a hand, and a shoulder to cry on...and in turn, I expect the same.

I have a group of friends in Chicago whom I see now only a couple of times a year, but each time we gather, it is always filled with laughter and conversation and love.
Two friends in particular in Chicago I count among my closest, Regina and Julie. Two totally different women, two tiny little women, but with huge hearts. I can talk to Regina when I'm having troubles and she listens and offers a kind word and support. It is just impossible to be down around Julie. She expects everyone to be happy and full of love. I'm smiling now as I think of them.

My two close friends in Dallas, Richard and Kevin. I've known them for 25 years and although I don't see them we still catch up with each other and our families.
But a friend I feel, correction, I know I have lost, leaves me baffled. But I cannot fix him and fixing is exactly what he needs but it must come from within. I mourn the loss because he was a good man...but I no longer know who he is now.