Hii, there is a fanfic that I don't remember its name, but I think Kurt had an accident and woke up thinking he was 17 years old (? It was something like that. But, I also remember that there was one exactly the same, but with Blaine being the one with the accident.
The Kurt amnesia fic is listed below. Check out the posts in our Amnesia!Blaine tag for ones with Blaine being in an accident. - HKVoyage
Let's Exchange the Experience by postweather
Thirteen years after they met, Kurt and Blaine are happily married and living in the city. Everything's perfect until Kurt gets into a car accident and wakes up thinking he's 17. He has no memory of his life after that, and all he knows of his relationship with Blaine is an unrequited crush. Kurt finds himself having to relearn the story of his entire life, and Blaine has to figure out how to help him.
Note: You need to be logged into an AO3 account to access this fic.
ETA: I think it could also be this story. ~Lynne
Everyday by whatIknew [PDF/EPUB]
A few days after their 2nd wedding anniversary, Kurt is in an accident and falls into a coma. Blaine struggles with the decision to let him go, and the night before they’re set to pull the plug, Kurt wakes up.
One problem: He thinks it’s 2008 and that he is 14 years old. No New Directions, no Finn and Carole, and no Blaine.
Blaine helps Kurt try to remember his old life while they consider starting a new one.