Spider-Man got his powers after being bitten by a radioactive spider. This gave him enhanced strength, agility, the ability to climb walls, and a 'spider sense'.
He might first go into denial. He could keep trying to do the things he used to do with his powers, like swing from building to building, only to fall. Then he could start to rely on his intelligence more. Since Peter Parker is a smart guy, he'd use his scientific knowledge to try to find a way to get his powers back. He may also look for allies who can help him, like other superheroes or scientists.
One reason could be a powerful magic spell cast on him by a villain. For example, a sorcerer who wants to stop Spider - Man from interfering in his evil plans might use a spell that disrupts the source of his powers. Another possibility is that he gets exposed to some sort of radiation or chemical that nullifies the mutation that gives him his powers. Maybe he enters a new dimension where the laws of physics are different and his powers don't work there.
Well, Spider Man often uses his powers in creative ways during his interactions. His agility allows him to easily navigate through a busy city street full of people without bumping into anyone. If there's a local fair, he can use his web to create a fun little ride for the kids, like a makeshift swing. His ability to climb walls can be useful when he's helping a window cleaner who's stuck on a high - rise building. He climbs up, rescues the cleaner, and then gives him some tips on safety. He also uses his powers to show off a bit sometimes, which always amazes the citizens and makes them cheer for him.
Well, in SI fanfics about Spider - Man powers, often we see the exploration of his spider - sense. It could be made more acute or perhaps have some new features added. And his web - slinging is another cool power. Some fanfictions might have him create new types of webs with different properties, like electrically charged webs for combatting enemies.
He could develop the ability to combine his chakra with his Spider - Man powers. For example, he could infuse his webs with chakra to make them stronger or even give them different properties like being able to cut through objects. Another new ability could be using his shadow clones in combination with his Spider - Man agility. He could send clones to different locations while swinging around with his webs, creating a more complex battle strategy.
His powers might not be affected much in terms of strength. However, the way he uses them could change. For example, his web - slinging might be a bit more graceful in his female form. Maybe his spider - sense could be more attuned to different things now, like more sensitive to emotions or subtler threats.
If Spider - Man had mutant powers in fanfiction, his origin story might be different. Instead of being bitten by a radioactive spider, he could be born with his powers. This would change his relationship with the scientific community. For example, he might not be as interested in science as the original Spider - Man. His powers could also be more uncontrollable at first, leading to different challenges in learning to use them.
His powers might be affected in different ways. For example, his web - shooting ability could be less precise. Since he's a baby, he doesn't have the same muscle control and coordination as his adult self. So, the webs might come out in a more random direction.