A group of thieves could be the kidnappers. They were planning on robbing Daria's house but found her there instead. They take her with them to avoid being caught and in the rush, her glasses are lost. They might keep her hostage until they figure out what to do with her, all the while Daria is trying to figure out how to escape and get her glasses back.
Perhaps it's a jealous rival. Maybe in the world of this fanfiction, there's some sort of competition, like for a job or a love interest. And this rival decided to kidnap Felicity to get her out of the way.
Another possibility is that a mysterious stranger comes to her rescue. This stranger might have been secretly following her kidnapper. He or she could fight off the kidnapper and then take Felicity to a safe place. And then they help her find a new pair of glasses.
It could be a group of rogue Death Eaters who think that kidnapping Harry would bring them back into Voldemort's good graces. They might take his glasses as a way to humiliate him, seeing Harry as the symbol of their defeat. They underestimate Harry's resourcefulness even without his glasses.
A plot twist could be that Harry's kidnapping and loss of glasses is the result of a time - travel mishap. A future version of himself (or someone else) accidentally causes the present - day Harry to be kidnapped and lose his glasses. The future version is trying to change the course of events for the better but ends up creating chaos in the present. Harry, without his glasses, has to figure out what's going on and how to fix the time - line while also escaping from his kidnapper. Another twist could be that the kidnapper is actually an undercover Auror gone rogue who is using Harry as a bait to draw out a much larger and more dangerous criminal organization. Harry's glasses are lost in the process, and he has to figure out who to trust among his captors.
The Avengers would be extremely concerned. Captain America would immediately start strategizing a rescue mission. Thor might be angry and ready to use his strength to find her. Black Widow would use her espionage skills to gather information on the kidnappers. And Iron Man would likely use his technology to track any possible leads, all while being aware that Darcy without her glasses might be in a more difficult situation.
Well, maybe he stumbles around blindly for a while, bumping into things and getting into all sorts of comical situations. He could also rely on his friends more to help him get around until he finds his glasses.
One possible consequence is that he might get injured more easily. Without his glasses, his vision is impaired, and being blindfolded makes it even worse. He could trip over things, bump into walls or other objects, and get bruises or cuts.