Kaze no Na wa Amnesia

The Apocalypse didn't end with a BANG, but with a whimper. Silently, the "amnesia wind" swept away all of mankind's knowledge. Thousands of years of human civilization vanished overnight as people forgot how to use the tools of modern civilization; who they were—how to speak—everything. Technology decayed as mankind was reduced to an extremely primitive level. Two years after the devastation, a young man explores a nation reduced to barbarism: America.

Miraculously re-educated after the cataclysm, a young man is accompanied by a strange woman—somehow spared the obliterating effects of the amnesia wind. Pursued by a relentless killing machine, they search for those responsible for stealing their memories.

(Source: Dark Horse)