Remus can't accept it. He won't accept it. Sirius can't just be gone - he wouldn't leave without saying goodbye. Not his Sirius. No, his Sirius was probably just hiding, finding Remus' abject and complete panic incredibly amusing. He would be behind the veil, ready to jump out with that fiendish grin and say,
"Aw, Did you miss me?" And Remus would be able to hug and hate him all at once for his cruel sense of humour. Then they'd go home together and be glad that they had both survived the most intense fight either had participated in since the first war. Sirius would probably give him a blow-by-blow account of his part in the fight, explaining how he had taken every Death Eater in the building down single handed. Remus would smile and nod then wrap his arms around Sirius' waist, rise up on his tiptoes just slightly and kiss Sirius' perfect lips...
A gentle shake of his shoulders snaps Remus from his trance. His eyelids feel heavy as they flicker open. He feels heavy all over - like his cloak is made of lead. He blinks blearily at the person in contact with him. His eyes meet the concerned gaze of Minerva McGonagall. She says something but he hears her as though through water – none of the syllables making words. Taking in his surroundings with a barely aware gaze Remus realises he is still kneeling in front of the veil. How he would like nothing more than to tear down that stupid piece of material - To rip it from the stone and catch Sirius in his hiding place. It would spoil the fun for Sirius but it had been hours now and it was no longer a laughing matter.
Yet deep down Remus knew that wasn't right. Sirius wasn't back there. When they had been younger boys they had played hide and seek round the castle. Sirius always elected himself seeker because he couldn't keep still longer than five minutes. He wasn't behind the veil. A wave of nausea swept through Remus. Sirius wasn't coming back.
He felt something torn away from him, something deep within his soul which he feels could never be replaced. He dropped his head into his hands and cried.