Listbox onselectindex event is not firing?

protected void GetByMonth(object sender, EventArgs e)

{

con = getcon.GetConnection();

int s = ListMenu.SelectedIndex;

cmd = new SqlCommand("select IncomeType,UserIncomeDetail_Amount from ft_UserIncomeDetail inner join Dim_IncomeType on ft_UserIncomeDetail.IncomeType_Id=Dim_IncomeType.IncomeType_id where MONTH(UserIncomeDetail_Date) = '" + s + "' and ft_UserIncomeDetail.User_Uniq='150883940'",con);

cmd.CommandType = CommandType.Text;

SqlDataAdapter sda = new SqlDataAdapter(cmd);

DataSet ds = new DataSet();

sda.Fill(ds);

Gridview_Bymonth.DataSource = ds;

Gridview_Bymonth.DataBind();

con.Close();

}