C# – ExecuteNonQuery requires an open and available Connection. The connection’s current state is closed

Your problem is:

SqlDataReader rdr = cmd.ExecuteReader(CommandBehavior.CloseConnection);

You should just call cmd.ExecuteReader()' if you want to use the connection again prior to "getting rid" of it. If you want to get an understanding for what the CommandBehaviour.CloseConnection part does/means then the documentation for SqlCommand.ExecuteReader is a good bet. There's also documentation to tell you what all the possible values of the CommandBehaviour enumeration are. Essentially CommandBehaviour.CloseConnection does the following:

When the command is executed, the associated Connection object is closed when the associated DataReader object is closed.

If you have no special need to specify a CommandBehaviour, then either specify CommandBehaviour.Default, or don't specify one at all. CommandBehaviour.Default is:

The query may return multiple result sets. Execution of the query may affect the database state. Default sets no CommandBehavior flags, so calling ExecuteReader(CommandBehavior.Default) is functionally equivalent to calling ExecuteReader().

You appear to be forgetting to open your connection (and are assigning the wrong connection to the command). Try:

       using(OleDbConnection conn = OpenDbConnection())
        {
          using(OleDbCommand command = new OleDbCommand( 
               "INSERT INTO tblUser (UserName, LoginName, Password) VALUES (@name,@login,@pwd)")) 
          {
          command.CommandType = CommandType.Text;
          command.Parameters.Add("@name", OleDbType.VarChar).Value = name; 
          command.Parameters.Add("@login", OleDbType.VarChar).Value = loginName; 
          command.Parameters.Add("@pwd", OleDbType.VarChar).Value = password; 
          command.Connection = conn; 

          conn.Open();

          command.ExecuteNonQuery();
          } 
        }

Instead. I'd recommend using the using statement, too. It will manage your connection.close for you.