How do I write a query to display the name(s) of the user who has/have answered most number of queries, sorted by name.

Your going to have to give a lot more information about what your desired output is. You are probably not seeing what you want as you've got a HAVING clause in your query. Otherwise your query looks like it is probably correct.

See example schema/data below

DECLARE @Users TABLE  (
	Id INT NULL,
	Username VARCHAR(200) NULL
);

DECLARE @Query TABLE (
	UserId INT NULL,
	Query VARCHAR(200) NULL
);

INSERT INTO @Users (Id,Username) VALUES (1,'user1')
INSERT INTO @Users (Id,Username) VALUES (2,'user2')

INSERT INTO @Query (UserId, Query) VALUES (1, 'A');
INSERT INTO @Query (UserId, Query) VALUES (2, 'A');
INSERT INTO @Query (UserId, Query) VALUES (2, 'A');
INSERT INTO @Query (UserId, Query) VALUES (2, 'A');
INSERT INTO @Query (UserId, Query) VALUES (2, 'A');


SELECT 
	A.Username,
	COUNT(*)
FROM @Users AS A
JOIN @Query AS B ON B.UserId = A.Id
GROUP BY A.Username
ORDER BY A.Username

Then the output of this is

Username	ItemsAnswered
user1	            1
user2	            4

Which is ordered by the username