One idea could be that during a mission crossover, Spider - Man gets injured while trying to protect a civilian from an unexpected attack by a powerful enemy. Maybe he takes a hit meant for an Avenger and his suit's technology malfunctions, leaving him vulnerable.
The Avengers would be very concerned. Captain America would immediately take charge of the situation, trying to keep everyone calm while they assess Spider - Man's injuries. Iron Man would start scanning for the best way to treat him, using his high - tech equipment. Thor might offer some of his Asgardian knowledge of healing, if there was any. Black Widow and Hawkeye would be on guard, making sure there were no further threats while the team focused on Spider - Man.
In a fanfic, Spider - Man could be injured while on a mission with the Avengers in outer space. Maybe they are exploring an alien planet and are attacked by some sort of bio - mechanical creatures. Spider - Man is separated from the group and gets severely injured by one of the creatures' acidic attacks. His injury makes it difficult for him to use his powers effectively, and the Avengers have to find a way to both get him back to Earth and find a cure for his injury.
He could be injured by a new and powerful villain that the Avengers and Spider - Man are facing together. The villain might have a weapon or power that can penetrate Spider - Man's suit and defenses, like a sonic blast that disrupts his spider - sense and then physically harms him.
Well, maybe he's got some secret mission that he doesn't want the Avengers to interfere with. You know, like protecting a particular person or some crucial information that he thinks the Avengers might mess up if they get involved.
Another possibility is that the Avengers are being manipulated by a villain. The bad guy could be using false information to make the Avengers believe that Spider - Man is a danger. So they go after him without really knowing the truth.
Spider - Man would likely be confused at first. He respects the Avengers and can't understand why they are suddenly after him. He might try to talk to them, to explain that he's innocent. For example, he could swing up to Captain America and start pleading his case.
Spider - Man would probably be shocked at first. He looks up to the Avengers and being kidnapped by them would be the last thing he expects. He'd likely try to talk his way out, using his quick wit and charm. He might start by asking why they're doing this and try to convince them that they've made a mistake.
Spider - Man would be very confused. He would feel betrayed because he thought the Avengers were heroes he could trust. He could start by asking them why they are doing this. If they don't give him a satisfactory answer, he might try to escape in a sneaky way, using his spider - like abilities to find a weakness in their capture setup while still trying to reason with them.
Spider - Man would probably be defensive at first. He's used to working alone most of the time, and being interrogated by the Avengers would seem unfair to him. He might crack some jokes to ease the tension, but also firmly state that he's a hero too.