1. Is there any fluid residue which could indicate the master or slave cylinders, or hose may have failed?
2. Are the bolts securing the slave cylinder and sprocket cover in place and tight?
2a. If the slave cylinder or sprocket cover bolts were loose, the slave cylinder may have popped of the bore, but it would also leave fluid residue.
3. You may need to pull the clutch cover and check the condition of the clutch springs and bolts.
3a. If ok, then remove the pressure plate and check the release bearing in the pressure plate.
These are about the only things I can think of that can cause a clunk with no lever pressure.