If you have already killed Drascua in the Companions quest "Kill the leader at Dead Crone Rock", when you reach her area, her corpse will still have the quest pointer above her head. As she is already dead, the quest is not able to be completed.
PC Fix: Using the resurrection console command can allow for the quest to be completed.
There may sometimes be a bug that causes Drascua to be invulnerable. Only sneak and cinematic attacks hit her, and afterward her body cannot be looted, while the quest objective marker remains hovering over the corpse. To fix this bug save, quit, and reload the saved game. (Tested on Xbox 360 only.)