The protagonist has lost or loses her powers, abilities, skills, spells, etc. intangible that make them presumably great and powerful. A notable part of the story is either spent reclaiming them or coping in their absence.
Alternate names: Lost levels, Lost abilities
The first video game about Lost powers was released on October 1991.
In RPG terms, player loses all or significant portion of their levels.
* Level drain attacks from "regular" monsters that can be somewhat trivially restored do not count. Nor do any other temporary status effects that prevent the protagonist from using all their potential.