In some fanfictions, when Harry Potter passes out from Dementors, he might be rescued by his friends. For example, Hermione could use a spell to drive the Dementors away and then revive him with a potion. Another possibility is that he has a vision while unconscious, perhaps related to his past or some hidden power within him.
One possible reason could be a powerful spell backfiring on him. In fanfiction, authors often play around with magic going wrong. Another reason might be extreme emotional distress. For example, if he discovers something truly shocking about his past or his friends.
His friends might be involved in some way. They could be trying to protect him but something goes wrong and Harry ends up passing out. For instance, they might be using a spell to shield him but it backfires.
After Harry passes out from Dementors in fanfiction, the plot may take a turn towards his character development. He might become more determined to fight against the forces of darkness. His friends could also become more protective of him. The story could then focus on training Harry to better defend himself against Dementors, leading to new magical skills or knowledge.
His passing out can also be a way to introduce new plot elements. Maybe when he's unconscious, he has strange dreams or visions that point to a new mystery or threat. This can then drive the rest of the story as the characters try to understand the meaning of these visions and prevent the potential danger.
It could also be due to extreme exhaustion. Harry often gets involved in intense battles or has to endure long periods of stress. For example, if he's been fighting multiple foes continuously without a break, his body and magic might give out, leading to him passing out.
It could be due to some kind of extreme emotional shock. For example, if he discovered something really horrible or had a sudden and intense encounter with an enemy, it might have been too much for him to handle and he passed out.
His friends Ron and Hermione would likely be the first to find him. They are always by his side in most situations. They know Harry well and would notice if he was in trouble.
Maybe he uses some powerful magic spells that he has learned secretly. For example, the Patronus charm could be used to fend off the Dementors which are the main guards in Azkaban. He might also get some unexpected help from his friends or some mysterious characters within the fanfiction world.
Gwen could be the one to find him. She often has a kind heart and pays attention to those around her. If she notices that Merlin has been looking overly tired lately, she might go to check on him and find him passed out. This could lead to her taking care of him and trying to figure out how to help him.