Has any one encounter before than when you issues df -h command but it hangs there and never comes out?
Even you open another session to kill it, although the kill command is successful, the pid still exist.
This is typical when a nfs share is not available any more.
When you have on your server a NFS share and its not available any more the df -h will hang for ever.
check with the mount command if you have an NFS share that is not available any more