This is SP2010 / NW 2010. I've got a library with a small handful of Nintex Workflow workflows bound to it. Until recently, I could access those workflows while browsing the libarary, by navigating to Library Tools > Library > Workflows > Workflow Settings. However, I no longer see any Nintex Workflow options in that Workflow Settings menu. I can, however, access the worflows at the Site Collection level by visiting Site Actions > Site Settings > Nintex Workflow Gallery.
I'm not sure why those Nintex workflow options disappeared from the Workflow Settings menu.