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.
In the West, we tend to affirm protest, intervention, resentment, and even revenge. In contrast, the Buddha tells us to let it go. That's quite a contrast to deal with.

AN5.162:6.10: In the same way, at that time you should ignore that person’s impure behavior by way of speech and body, and the fact that they don’t get an openness and clarity of heart from time to time, and think of them with nothing but compassion, kindness, and sympathy:  ‘Oh, may this person give up bad conduct by way of body, speech, and mind, and develop good conduct by way of body, speech, and mind.  Why is that?  So that, when their body breaks up, after death, they’re not reborn in a place of loss, a bad place, the underworld, hell.’

When the behavior of others disturbs us, we must first acknowledge that the perception of disturbance is within us--we see ugliness to which feel aversion and experience a desire to &quot;make it right&quot;. Propelled by this aversion and desire, our resulting actions are corrupted by our own aversion and desire. Because of this, we generate suffering for ourselves with delusions of righteousness.
So our first action is to deal with the mess inside, clean it up and let it go. If our feelings are hurt, we let that go. If we feel resentment, we let that go. If we feel anger, we let that go. We have to let all the clinging go. Because if we don't let go, our actions will have an unskillful root. Such actions always lead to more suffering.
Relationships have a beginning. And because they have a beginning, they also have an end. Thoughtless endings, ghosting and worse, hurt both parties. It is sad to treat each other like an Amazon shopping experience. We become each other's dismissable clickbait. Yet perhaps a thoughtless ending was matched by a thoughtless beginning. Perhaps we can consider and nurture relationships that matter.
The Buddha discusses the danger of fake friends, and urges us to be wise in the relationships we start and maintain:

DN31:15.1: “Householder’s son, you should recognize these four enemies disguised as friends: the taker, the talker, the flatterer, the spender.

Fake friends aside, we can look to become a good friend worthy of good friends:

DN31:21.1: “Householder’s son, you should recognize these four good-hearted friends: the helper, the friend in good times and bad, the counselor, and the one who’s compassionate.

Let go of bad relationships. Open your heart to good-hearted relationships. And keep your heart open even to people from bad relationships--there may come a time when such a person comes back to you to make amends. Be open to that.