How do you fix the bug in skyrim where karliah wont talk?

To fix the bug in Skyrim where Karliah won't talk during the "Darkness Returns" quest, you can try reloading a previous save just before the quest starts. Alternatively, using console commands (on PC) can also help: open the console with the tilde key (~) and type setstage TG08 200 to advance the quest. If you're on a console, you may need to try exiting and re-entering the area or waiting a few in-game hours to reset her behavior.