couldn’t add user : there are no users authenticated
It is because you already had a user configured. You should go back and read about the localhost exception for creating users.
usually when i display admin collection i found that i have user created , right now i cant
Once you have a user configured, the localhost exception is closed, you have to authenticate to do anything.
As @steevej-1495 mentioned, you have already created a user and now you are required to authenticate yourself before you can perform any operation.
You can use db.auth() method to authenticate or you can also append the username and password at the end of your connection string.
Hope it helps!