Data Controls :: Connection String Is Not Initialized Error

public class DataClass{    public DataClass()    {    }    /// <summary>   
///  return rows depend on position    ///  if you need 10th to 20th you need to pass start=10 and end=20   
/// </summary>    /// <param name="start">database start position of one row</param>   
/// <param name="next">database end position of one row</param>   
/// <returns></returns>    public string GetAjaxContent(int start, int end)    {          

[code].....

When I'm trying to develop the unlimited scrolling in datalist i have an error like connection string not initialized I checked and rechecked.