http://www.jb51.net/os/Ubuntu/164636.html
1, restart
2, Hold down shift key / press and hold
3, select "recovery mode"
4, select "root Drop to root shell prompt"
5, check username : cat /etc/shadow
6, :passwd "username"
error: c“-passwd：Authenticationtoken manipulation error“
reason: recovery mode is read-only filesystems,
check authority : lsattr /etc/shadow
solution: change file systems authority : mount / -o rw,remount