This is a known bug with the Ghostblade, unfortunately. Are you sure it's actually gone, and not just invisible? Try moving your pointer around the rack and see if it's registers as if it were an invisible "dropped" item. If you're playing on a console (360/PS3), your only other options are 1) accept its loss, or 2) reload a previous save, unfortunately. If you're on the PC, it's an easy fix via console commands (let me know and I can tell you what to do).