How does Ponyboy react to Johnny’s death?

I think that Ponyboy reacts in a way that is similar to how many people react when a close friend or family member dies unexpectedly. Ponyboy simply can't bring himself to believe and accept the fact that his friend is dead. Johnny dies at the end of chapter 9, and chapter 10 sees Ponyboy denying the fact that Johnny has just died. Deep down Ponyboy knows it is true, but he can't bring himself to fully understand and embrace the concept:

Johnny was dead. But he wasn't. That still body back in the hospital wasn't Johnny. Johnny was somewhere else -- maybe asleep in the lot, or playing the pinball machine in the bowling alley, or sitting on the back steps of the church in Windrixville. . . He isn't dead, I said to myself. He isn't dead. And this time my dreaming worked. I convinced myself that he wasn't dead.

Even after Ponyboy tells the other Greasers that Johnny is dead, Ponyboy still inwardly tells himself that Johnny isn't dead. Ponyboy turns even more inward than normal, and he ends up staying fairly inactive for another week. He simply can't bring himself to focus on anything, but his denial is still strong. Randy comes over to Ponyboy's house in chapter 11, and Ponyboy once again states that Johnny isn't dead. Even throughout most of chapter 12, Ponyboy simply isn't moving on and dealing with the fact that Johnny is dead. Ponyboy's grades begin slipping, and he is quite irritable. It isn't until Ponyboy finds the note Johnny left him that Ponyboy starts to come out of his mental fog.

When Johnny dies at the end of chapter 10, Ponyboy and Dallas are right next to him in his hospital room. Dallas reacts to Johnny's death with violent emotion, slamming his body and his fists against the wall of the room, while Ponyboy reacts with silence and shock.
Chapter 11 opens with a description of Ponyboy's experience. He uses words like "daze" and "stupor" to describe his state of mind. As well, Ponyboy's thoughts take him through imaginings of Johnny while Johnny was still alive; he imagines Johnny playing pinball and sitting on the steps of the church, for example. Ponyboy explains that his dreams and his imagination are able to convince him that Johnny isn't dead. Ponyboy's denial of Johnny's death is a reflection of Ponyboy's pain and sense of loss. Unfortunately, the reader knows that Ponyboy's dreams of Johnny and his sense that Johnny must still be alive are only temporary; soon, the reality of the situation will hit Ponyboy, and he will suffer his loss deeply.