session for login and logout button

Logout button:-

string username = Session["uname"].ToString();
string password = Session["pwd"].ToString();
lbl_1.Text = username;
cn.Open();
SqlCommand cmd = new SqlCommand();
cmd.Connection = cn;
cmd.CommandType = CommandType.StoredProcedure;
cmd.CommandText = "SP_TRACK_USE";
cmd.Parameters.AddWithValue("@Username", username);
cmd.Parameters.AddWithValue("@Password", password);
cmd.Parameters.AddWithValue("@COMMAND", 1);
cmd.ExecuteScalar();
cn.Close();
Response.Redirect("Default.aspx");

and the store procedure for this code :-

ALTER PROCEDURE [dbo].[SP_TRACK_USE]

@Username varchar(50),
@Password varchar(50),
@COMMAND INT = 0
AS
BEGIN
SET NOCOUNT ON;
IF(@COMMAND = 0)
BEGIN
INSERT INTO USERLOG (Username,Password,LOGIN,LOGOUT) VALUES (@Username,@Password,GETDATE(),NULL)
END
ELSE IF (@COMMAND = 1)
BEGIN
UPDATE USERLOG SET LOGOUT = GETDATE() WHERE Username=@Username and Password=@Password
END
END

I want that when user click the logout button then it redirect on login page.
My code redirect to login page without any error but not save the logout time in database.