Mongo authentication issues
You should not need to run mongod again if it is already running, and chances are that it already is. You are also not saying how you are trying to connect, which is likely your problem.
I would suggest reading the relevant documentation which explains what the default user authentication is and how to connect:
$ mongo admin --username root --pasword YOURPASSWORDWhere the default password is contained in the documentation page. There is also information on setting up new user for your application.
For more information, see the official MongoDB documentation which has many examples:
http://docs.mongodb.org/manual/administration/security-access-control/
You are able to successfully connect to database but you does not have admin privilege for this.
If you know the admin user id and password,You can authenticate by
db.auth("user_name",'passwd');