This is a question mostly asked by those who have only seen the anime and it is a big one, how Kaneki lost his memory?
Kaneki lost his memory after the fight against Arima (CCG) where he pierced both his eyes. Thanks to Kaneki's regenerative abilities, his brain rebuilded itself to allow him to survive, however his memories couldn't be "regenerated" and were, unfortunately, lost.
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How did Ken Kaneki lose his memory ?
During the operation against the Owl (chapters 138 and 139 of the manga), the CGC attacks the Anteiku coffee where the ghouls are found. Hide, Kaneki's childhood friend, was part of the operation on the CGC side.
Unfortunately Hide was fatally injured during this operation. Kaneki and Hide have one last discussion before Hide loses consciousness. Being his only real friend, Kaneki loses his mind after his death and tries to attack Kishou Arima, one of the CGC's best agents.
Kaneki vs Arima
Arima Kishou fights Kaneki Ken, and wins. He pierces Kaneki's eyes with his quinque, blinding him. The tip of the quinque also pierces the brain. A normal human would of course have died from this wound, but Kaneki heals thanks to his regenerative faculties.
However, these faculties allow the brain to be rebuilt, but do not allow memories to be "regenerated". C'est pourquoi Arima le prend sous son aile et en fait un agent de la CCG, ce qui constitue le cadre de l'histoire de Tokyo Ghoul:Re.
Does Kaneki regain his memories ?
Yes, don't worry, Kaneki get his memory back in Tokyo Ghoul Re, thanks to Hinami's efforts.