Without having it in front of me I couldn't say. I'd hazard a guess that the tailpiece has seperated or broken, so the cylinder isn't actually interacting with anything. If it's in working order then maybe you're turning the wrong way, although in saying that, it shouldn't turn completely in the wrong direction. Assuming the door is both closed and locked, and the mech is faulty, then your best bet would be to call a locksmith. If the door is open, take the lock out of the door and investigate.
Edit: Said spindle instead of tailpiece for some reason. Heh.
Also, we have an edit button now? Awesome!
