Ubuntu 12.04 ‘can not lock /etc/shadow try again later’

I can not log with my password nor can I change my password with root.

Others have met with the problem too.

https://ask.fedoraproject.org/question/1518/fc16-adduser-cannot-lock-etcpasswd-try-again-later

http://superuser.com/questions/296373/cannot-lock-etc-passwd-try-again-later

https://answers.launchpad.net/ubuntu/+source/shadow/+question/174969

 I entered recovery mode and get the authority of root.

 Then I deleted the .lock files from /etc folder (/etc/.passwd.lock, /etc/group.lock, …)

Then it works well.