He could also become angry. Percy has a bit of a temper, and he might lash out at Annabeth, demanding an explanation. He might feel betrayed, especially if he doesn't understand the real reasons behind the break - up. In his anger, he could distance himself from their mutual friends and go on solo quests to clear his head.
On the other hand, Percy could be heartbroken. He might withdraw into himself, spending a lot of time alone by the ocean, which has always been a source of comfort for him. He would be constantly thinking about what went wrong in their relationship and might even blame himself for not being a better partner.
At first, Percy could be in denial. He might think that there's some misunderstanding, that Annabeth doesn't really mean to break his heart. So he tries to talk to her, to make things right. But as the truth sinks in, he could be really depressed. He might lose his appetite, not be able to sleep well, and just generally seem like a shell of his former self. His whole world revolved around Annabeth, and now that it's falling apart, he doesn't know how to cope.
The campers at Camp Half - Blood would be shocked. Some might take sides, especially those who are closer to either Percy or Annabeth. The Aphrodite cabin might be all up in the drama, trying to play matchmaker again to fix things. The Ares cabin might see it as a sign of weakness in Percy and try to pick on him a bit more. Overall, it would create a lot of gossip and tension around the camp.
Percy might be a bit flustered and embarrassed. He's protective of his relationship with Annabeth, so having Magnus walk in might make him feel like their privacy has been invaded. He could quickly try to shield Annabeth or just look really shocked.
He could also become withdrawn and depressed. Percy and Annabeth have a very strong bond, and her death would leave a huge void in his life. He might isolate himself from his friends and family, lost in his grief and memories of her.
He would probably be shocked. Percy has always trusted Annabeth, so being stabbed by her would be completely unexpected. He might initially be unable to believe what's happening.
At first, Percy would likely be angry. He's a fighter, and being betrayed by someone he loves would fuel his rage. He might go after Annabeth to demand an explanation. But as the reality sinks in, he could become very sad and withdrawn. He might isolate himself from his other friends as well, feeling like he can't trust anyone anymore. He would constantly be thinking about what he did wrong to make Annabeth turn on him.
Well, in many such fanfictions, Percy might be extremely flustered. He could blush furiously and quickly look away, being the gentleman that he is. After all, he has deep respect for Annabeth.
Percy could be furious. He has always been loyal to Annabeth, and this betrayal would be a huge blow. He might immediately cut off all contact with her and go off on his own for a while to cool down.
Percy would feel a deep sense of guilt as well. He might think that he should have been there to protect her. So, he would be determined not only to free her but also to make sure whoever is responsible pays for what they've done. He would use his powers over water to his advantage, like creating distractions or using water to weaken the enemy's defenses while he tries to reach Annabeth.
She might be heartbroken at first. She could try to jog Percy's memory by showing him things that were important to them.