Internal server error 500

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ASP.NET4

Hi,
i am using following code in xx.svc and i called Login Methode through ajax in aspx page.. but it gives sometimes Internal server error 500.. How to resolve this problem.. anybody know pls help me..

[OperationContract]
              [WebInvoke(Method = "POST",
                  BodyStyle = WebMessageBodyStyle.WrappedRequest,
                  ResponseFormat = WebMessageFormat.Xml
              )]
              public string LogIn(string UserID, string Password)
              {

                  try
                  {
                      string strUserId = UserID;
                      string strPassword = Password;
                      int rVal;
                      Hashtable inParams = new Hashtable();
                      inParams.Add("@uid", strUserId);
                      inParams.Add("@Password", strPassword);
                      string result = "";
                      DataSet objdetDataSet = DataHelper.GetDataSet(connString, "CheckUserExists", out rVal, inParams);
                      if (objdetDataSet.Tables[0].Rows.Count > 0)
                      {
                          result = objdetDataSet.GetXml();


                          System.Web.HttpContext.Current.Session["UserId"] = objdetDataSet.Tables[0].Rows[0].ItemArray[0].ToString();
                         
                        
                      }

                      return result;

                  }
                  catch (Exception ex)
                  {

                      return ex.Message;
                  }
                  finally
                  {
                  }
              }