After having prepared for the war for so long, the seven couldn't imagine loosing one of their own. They were prepared. They could fight. They were going to win.
But now the war is over. The demigods won, but they lost one along the way. Jason. His best friends are destroyed by this.
Piper's in complete distress in her own destroyed world, and Leo denies the truth with every bang of his hammer. But the truth, complete with the pain, misery, and grief it brings will eventually sink in, leaving the two gasping for air.
But along with the other four, they can make it through this catastrophe. They can make it to see the light of another day.