They shoo him away, too caught up in being reunited, being free, the apocalypse being reduced to a bad memory. They shoo him away, they don’t call for him they don’t do anything to contact him, and why should they? After all he is not a human, he isn’t their family, in the end all he sacrificed hasn’t brought him close enough to them that they will love him.
He looks at the sky, and he calls out again and again, to the only brother he can think off that might not smite him, or desert him, he calls out to Gabriel, because some strange feeling in his chest tells him he should call for Gabriel, he is alive, Castiel knows this, but he receives no answer, it seems that his brother doesn’t care either, he has no one left, nothing to fight for, no one to help, and it hurts, hurts because he gave up everything for everyone else and they cast him aside when he was not needed anymore, ignored him and forgot him as soon as the war was over.
He doesn’t remember ever crying before, after all he have never truly known feelings have he? But now, now with his wings withering off his back, dark feathers falling around him, he himself falling from grace with no one to catch him, no one that cares enough to help him in his pained state, now he feels, feels as he breaks apart, desperate and hopeless.
He stands here alone, for how long he has been here staring out over the ocean, dark clouds filling the sky, wind and rain blurring the line between the water and sky, silently crying, so hopeless and so sad that his remaining grace grows colder and colder, and he screams to the heavens, screams to the father who was the first to abandon him, screams because he can’t take this, he was never meant to feel, was he? It is tearing him apart.
Death will do him no good, he would for eternity wander in purgatory, still heartbroken and in pain, but he can’t live on either, can’t live and cant die, so he begs for a miracle, begs to escape this, escape this existence that has become a plague, and his wish is granted, his remaining grace becomes like crystal, hard and cold and he just stands there as a coffin builds around him, makes him a pillar of clear hard grace, nothing will break this, and he will sleep here forever, nothing to harm him nothing to feel, suspended in a dreamless state. He is still crying as the crystal builds around him, in the end there will be nothing. This is the only comfort for the falling angel as he takes a last shaky breath and becomes cold.
In truth when he thinks about it took them way too long to realize that Castiel was gone, they were too caught up in the happiness of seeing each other again, too preoccupied with celebrating their victory that they forgot the angel who helped them so much, the angel who saved them, but when he realizes it is far too late, no matter how often he calls for the angel, no matter how much he prays Castiel never shows, and Dean knows, in his heart and in his bones that something is wrong, Castiel was falling, even after the war was won the angel must have continued to fall, and it hurt to think about the pain he must have been in, maybe still is in, and then have no one to help, because Dean was selfish, because he only had eyes for his brother and forgot about the angel, so Dean prays harder, he tries everything he can think off, tries to summon Castiel, he even tries the angels cellphone, but nothing works, there’s no answer. Sam too tries with all his heart to find the angel, he too is calling out, with prayers and his voice, but there’s no answer, no angel, and the brothers cry, silent tears they do not show, because they feel it in their souls, the regret the shame, that they drove the angel away.
He heard his brother call out to him, sensed his brothers pain, but he never answered, thinking "You have Dean and Sam now little brother, go talk to them", but the moment he felt the cold he knew he had made a mistake, he felt his brother’s sorrow and head his brother’s screams, felt his brother fade away, and he hurried to earth, to the first place he thought he would find his brother, but all Gabriel found was nothingness, darkness, so he searched, feeling after his brothers ever fading grace, but found nothing, he searched high and low but found no sign of his brother anywhere. So he returned to heaven, to search for his brother there, but here too he found no trace, no word of his brother, so he returned to earth, he returned to the Winchesters, to ask them about Castiel, but what met him was no answers, what met him was men as desperate as he, they knew nothing, they too were searching, so they joined forces, combing the earth for the slightest clue as to where the falling angel might be.