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.
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.
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.
Ginny might initially be heartbroken. She could retreat into herself, feeling betrayed and confused. In some fanfictions, she may cut off all contact with Harry immediately.
Ginny could be heartbroken. She might initially be in denial, thinking that Harry will change his mind. She has loved Harry for a long time and expected a future together, so this refusal would be a huge blow to her emotions.
Ginny could be heartbroken. She might have expected to marry Harry after all they've been through together. She could initially go into a state of shock and disbelief, not understanding why Harry has refused.
She could also be very hurt and go into a state of deep sadness. She might isolate herself from Harry and their mutual friends for a while. Ginny would likely question her own self - worth and wonder what she did wrong in the relationship. But as time passes, she might find the strength to move on and focus on her own life, like her career or other relationships.
Harry might immediately rush to her side and comfort her. He could put his arm around her and ask her what's wrong in a gentle voice.
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 would most likely be extremely happy. She has deep feelings for Harry, so his proposal would be a dream come true for her.
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.