Seriously this opens up so much creativity for the DM.

"This person seems familiar to you, but you're not sure why."

"You think you've been here before, but you're not sure when."

"Being on a ship feels like home, but you have no memories of ever being on a ship."

"Upon seeing the ruins of the village you find yourself overcome with sorrow, you feel like you've lost something important but you don't know what."

Part of the adventure can be a quest to reunite them with their lost memories.