Kaze no Na wa Amnesia  

A Wind Named Amnesia, Wind of Amnesia, A Wind Named Amnesia/Invader Summer, 風の名はアムネジア

The end of the world did not occur with a loud explosion, but rather quietly as an "amnesia wind" swept away all human knowledge. In just two years, civilization crumbled and people forgot how to use modern tools. Language, identity, and technology were lost. However, miraculously, a young man was able to re-learn after the disaster. Along with a woman who was spared from the amnesia wind, they embark on a journey to find those responsible for stealing their memories and bringing about the end of civilization as they know it.

Kaze no Na wa Amnesia was published in English as A Wind Named Amnesia/Invader Summer by Dark Horse in 2009.