Her friends would be extremely worried. Harry and Ron, especially, would be very concerned about her safety. They might start searching for her immediately, thinking that she could be in some kind of trouble or danger. They know Hermione well and understand that she wouldn't run away without a good reason.
On the positive side, it could bring her friends closer together as they try to find her. Harry and Ron might put aside any petty differences they have and focus solely on bringing Hermione back. They might also discover new things about Hermione's life and family during their search, which could strengthen their friendship in the long run.
Perhaps he was influenced by some new friends he met who had a different lifestyle or view of the world. They might have convinced him that running away from home was an exciting adventure and a way to break free from the restrictions at home.
One possible reason could be that she feels misunderstood at home. In the wizarding world, Hermione is a very intelligent and studious witch. If her family doesn't understand or support her magical pursuits, she might feel the urge to run away. For example, if they constantly tell her to focus more on 'normal' things and less on her spells and potions, it could drive her away.
He might first find a place to hide, like an abandoned building. There, he starts to realize how difficult it is to be on his own. He has to find food and water by himself.
If Hermione runs away from the Order, she might seek out ancient magical knowledge. There are many hidden libraries and places in the wizarding world filled with knowledge long forgotten. She could be on a quest to find such places and use the knowledge to combat the Dark forces in her own way.
One reason could be she feels extremely pressured by the high expectations at Hogwarts. Everyone expects her to be perfect in academics and problem - solving. Another might be that she discovers some dark secret within the school that she's afraid of, like a hidden plot by a group of dark wizards among the staff or students. Or she could be running away to protect her friends. If she believes that her presence at Hogwarts endangers Harry and Ron, she might choose to leave to keep them safe.
It could cause a sense of loss. Harry is a symbol of hope for the wizarding community. His running away might make some people feel abandoned.
She could meet other outcasts from the wizarding world. There might be witches and wizards who have left Hogwarts for similar reasons and they form a sort of community in hiding. Maybe she'll meet a powerful but reclusive wizard who can teach her things that Hogwarts never could.
Another reason could be that she wants to explore the world on her own terms. In the fanfiction, Mirabel may be tired of always being in the family's protective bubble. She desires to see what lies beyond the village and the mountains, so she runs away to start her own adventure.
For Harry, it could be a huge emotional blow. Sirius was like a father figure to him. His running away might make Harry feel abandoned again, and it could also fuel his determination to find out the truth and bring Sirius back.
He might miss out on some crucial training. Jiraiya has a lot of knowledge and skills to pass on, and by running away, Naruto could fall behind in his development as a ninja.