Here's a few examples start out!
>You lost your compass and must turn back.
>You are in danger of falling off the edge of the world and must turn back.
>The Nine Divines withhold your progress in that direction.
>You trip and fall clumsily and must turn back in shame.
>You cannot trust anyone outside the country and are forced to turn back.
>You find the hidden magical barrier of the lost wizard Sladus and are forced to turn back
>You hit an invisible wall.