It could be a jealous classmate from Hogwarts. Someone who has always been in Harry's shadow and finally sees an opportunity to get rid of him. This person might be a Slytherin who has connections with some dark wizards outside of school and they work together to frame Harry.
Well, it could be many reasons. Maybe there was a misunderstanding. Some dark magic might have been used to make his friends think he did something bad. Or perhaps a powerful enemy framed him by planting false evidence. It's hard to say exactly without more context from the fanfiction.
Harry might survive in Azkaban because of his connection to his friends, even if they betrayed him. Their memories could give him the strength to hold on. And he may be able to use his magic in unique ways. He could draw on the positive emotions he has left, like his love for his parents, to create a shield against the Dementors. Plus, he may be constantly looking for a way to prove his innocence and that hope could keep him alive.
One possible plot could be that Harry is framed for a serious crime he didn't commit. Maybe someone has used Polyjuice Potion to look like him while doing dark magic. The wizarding community, believing the false evidence, sends him to Azkaban. In Azkaban, he has to endure the Dementors and try to prove his innocence from within the prison walls.
It could be Dumbledore. He has great power and knowledge. He might have some secret plan all along. He could use his influence and magic to enter Azkaban and free Harry, especially if he discovers that Harry is wrongly imprisoned.
In this fanfiction, Harry might meet students from various backgrounds. There could be students from far - off magical lands who have their own unique cultures and magic. He could also run into some Beubaxtons alumni who come back to visit and they might share some interesting stories or give him some advice on how to make the most of his time at Beubaxtons.
He could meet his future self. This would be a very interesting encounter as he could learn from his future self about what he has accomplished and what mistakes he should avoid. He might also meet the future generations of his family, like his grandchildren or great - grandchildren.
Perhaps Harry will run into an old wizard who has been in the hospital for a long time. This wizard has a lot of stories about the old days of the wizarding world and gives Harry some valuable advice.
Hermione might be one of the helpers. She's very intelligent and could come up with a clever plan to get Harry out. Maybe she uses her knowledge of ancient spells.
Obviously, Harry Potter himself is a main character. Hermione and Ron often play important roles too. They might try to break Harry out of Azkaban or help him from the outside. Sometimes, Sirius Black could also be involved, especially if the story is related to the secrets of Azkaban or the wrong imprisonments. Dumbledore could also be a key figure, guiding Harry or influencing the events from afar.
Hermione could be the one. Maybe she gets jealous of Harry's fame and power, and starts spreading lies about him, which leads to his betrayal. As a result, Harry feels so hurt that he turns to Voldemort.