Amnesia

Amnesia is a condition that results in the loss of memory caused by brain damage, psychological trauma, or a disease.

History
Medusa developed amnesia when she left the Great Refuge. She was convinced by Wizard to join the Frightful Four and fight the Fantastic Four. She eventually remembered who she was when she rejoined the other Inhumans.

Spider-Man got amnesia during a battle with Doctor Octopus. He got his memories back thanks to Maria Taina Elizando.

Bruce Banner, and thusly Hulk, developed amnesia after an overpass collapsed and he hit his head. While transformed as Hulk, he remembered who he was, thanks to Rick Jones.