I am trying the Mongo Atlas Cloud. I create a cluster and i am trying a connection with the mongo shell: (same problem with mongo drivers)
mongo mongodb://***-cl
I also faced the same issue. "admin" user would be created at the time of db creation. If you forgot the password for the default admin user, then it is difficult to connect with the database and access the collections.
You could create one more database user under "Security->Database Access" section and select "Atlas admin" as a role.
If your password contains any special character, then this would be encrypted internally. Hence, use this link (https://www.urlencoder.org/) to encrypt your password (plain text) and use the encrypted string to establish a connection.