When you notice people you cared become distant from you, what do you do? And what are your thoughts?

Usually, I reach out a couple of times just be sure that they didn't just miss a phone call or email. That's happened. I'm talking about long-time, once close friends.

If I don't hear back from them, I just accept that they aren't my friend anymore. I wonder why...what changed. I stop 'reaching out', even though I think of them and wonder how they are. It hurts.