How to let go of something the other person involved won't acknowledge

How does one let go of something the other person involved won't acknowledge? I don't mean the niceties of the other person's behaviour, the ins and outs and whys. Merely, that the other person - I think - nihilistically won't finish up. So e.g. your lover leaves you without even leaving a note. Or your friend pretends not to know you, with no explanation. Or your family changes the locks and won't answer the door when you visit.