At first, Ginny would be hurt and disappointed. But being Ginny, she'd also be a bit defiant. She might act like it doesn't matter that much in front of Harry, but then go and pour her heart out to Hermione. She'd be thinking about all the times she remembered important things for Harry, and feeling a bit let down. And she might start to distance herself from Harry until he makes it up to her.
Harry would probably be very confused at first. He's so used to Snape's animosity towards him that when Snape forgets who he is, it would be a complete shock. He might also be a bit suspicious, thinking it's some sort of trick. But as time goes on, he could start to feel a bit of pity for Snape, especially if Snape seems lost or confused without his memories of Harry.
Well, it could start with Ginny waiting all day for Harry to remember. When he doesn't, she gets angry and ignores him. Harry, being a bit clueless, doesn't understand why she's so mad until Hermione smacks him on the head (figuratively) and tells him. Then he might plan a surprise party in the Room of Requirement with all of Ginny's favorite things, like butterbeer and Weasley sweaters in different colors.
Ginny would likely be protective. She loves Harry and knows about his difficult past with the Dursleys. So she would probably stand up for him right away, not tolerating any of their mean remarks.
Ginny might be in disbelief for a moment. She has always seen Harry as this heroic figure and now he's asking her to the ball. But then her natural confidence would take over and she would accept with enthusiasm. She would also tease Harry a little bit about taking so long to ask her, adding a bit of humor to the situation.
Harry would probably be heartbroken. He trusts Ginny deeply, so her betrayal would be a huge shock. He might initially be in denial, not wanting to believe that Ginny could do such a thing.
She might also be sad and heartbroken. Ginny is a strong character, but being left alone while pregnant is a huge blow. She could withdraw from her friends and family for a while as she tries to come to terms with the situation and figure out how to raise the child on her own.
At the start, Ginny would be shocked and terrified when Tom Riddle kidnaps her. However, as time passes, her Gryffindor courage would kick in. She would start to analyze Tom Riddle's motives and actions. She might pretend to be more vulnerable than she actually is to lower his guard. Ginny could also try to communicate with any magical creatures or objects in her vicinity for help. And she would be mentally preparing herself for a fight or escape, using every bit of her magical training and her wits to outsmart Tom Riddle.
Sakura would be heartbroken. She'd likely spend hours upon hours by Naruto's side, trying to jog his memory with stories of their adventures together. She might even get a bit overbearing in her attempts. Sasuke, who has his own complex relationship with Naruto, would be conflicted. He'd see Naruto's forgetfulness as a sign of weakness at first but then realize the importance of their bond. Kakashi, being the experienced leader, would start by observing Naruto carefully to see exactly how much he has forgotten and then start a systematic process of helping him remember.
Their other friends in Gryffindor would be quite shocked. Neville, for instance, might stutter and not know how to react. Luna might have a more calm and accepting reaction. She might say something like 'This is a very interesting turn of events. I wonder what the magical cause could be?' The Weasley family would be especially confused. Molly Weasley might be worried about Ginny's well - being and Arthur Weasley would be trying to figure out how such a strange magical occurrence could have taken place.
He could also be secretly amused. Despite his stern exterior, Snape might see a bit of his own unrequited love in their relationship. So, he might give them a stern warning but not be as severe as he usually is.