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
Issue 'umountall -r'. This command will unmount all NFS filesystems, if you cannot determine which NFS share is not responding. Check also /var/adm/messages for related NFS errors.
Edited by: Reidod on Apr 30, 2013 12:30 PM