Code:
using System;
using System.Data;
using System.Configuration;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
using System.Data.SqlClient;

public class Helper
{
    static public DataSet GetDataSet(string strSql)
    {
        DataSet ds = new DataSet();
        SqlConnection connection = new SqlConnection(ConnectionString());
        SqlCommand command = new SqlCommand(strSql, connection);
        SqlDataAdapter dataAdapter = new SqlDataAdapter(strSql, connection);
        dataAdapter.Fill(ds, "tblUsers");
        return ds;
    }
}
and i get this error:

Code:
CS0246: The type or namespace name 'DataSet' could not be found (are you missing a using directive or an assembly reference?)
It used to work i dont know what heppend...