Postback doesn't work after Gridview binding

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ASP.NET

Ajax

Gridview

C#5.0

In my ASP.NET Application, I have a dropdownList which is used to load form Data from SQL Server. Everything was working perfect until I bind the gridView. After the gridView is bound, No postback will happen in the page.

This is my code.

protected void DropDownList3_SelectedIndexChanged(object sender, EventArgs e)
     {
     using (SqlConnection con = new SqlConnection(System.Configuration.ConfigurationManager.ConnectionStrings["ApplicationServices"].ConnectionString))
     {
         con.Open();
         //using (SqlCommand com = new SqlCommand("SELECT CountryID, (CASE ISNULL(CountryLOC, '') WHEN '' THEN '' ELSE  ISNULL(CountryLOC, '') + ' - ' END) + CountryName AS CtrName FROM Country ORDER BY CountryName", con))
         using (SqlCommand com = new SqlCommand("SELECT * FROM Employee WHERE EmployeeID = " + DropDownList3.SelectedValue, con))
         {

             SqlDataReader dread = com.ExecuteReader();
             if (dread.Read())
             {
                 //main
                 firstName.Text = dread["FirstName"].ToString();
                 middleName.Text = dread["MiddleName"].ToString();
                 lastName.Text = dread["LastName"].ToString();
                 nickName.Text = dread["NickName"].ToString();
                 legalFName.Text = dread["LegalFName"].ToString();
                 legalMName.Text = dread["LegalMName"].ToString();
                 legalLName.Text = dread["LegalLName"].ToString();
                 legalMaidenName.Text = dread["LegalMaidenName"].ToString();

                 //General tab
                 dateOfBirth.Text =(dread["DateOfBirth"] != DBNull.Value ? Convert.ToDateTime(dread["DateOfBirth"].ToString()).ToString("dd/MM/yyyy") : "");
                 maritalStatus.Text = (dread["MaritalStatus"] != DBNull.Value ? dread["MaritalStatus"].ToString() : "-1");
                 genderMale.Checked = (dread["Gender"].ToString().Trim() == "M" ? true : false);
                 children.Text = dread["NumberOfChildren"].ToString();
                 homePhone.Text = dread["HomePhone"].ToString();

                 mobile1.Text = dread["Mobile1"].ToString();
                 mobile2.Text = dread["Mobile2"].ToString();
                 email.Text = dread["PersonalEmail"].ToString();
                 address1.Text = dread["AddressLine1"].ToString();
                 address2.Text = dread["AddressLine2"].ToString();
                 city.Text = dread["City"].ToString();
                 poBox.Text = dread["POBox"].ToString();
                 state.Text = dread["State_Province"].ToString();
                 zipCode.Text = dread["Zip_PostalCode"].ToString();
                 country.Text = (dread["CountryID"] != DBNull.Value ? dread["CountryID"].ToString() : "-1") ;
                 nationality.Text = (dread["Nationality_CountryID"] != DBNull.Value ? dread["Nationality_CountryID"].ToString() : "-1");

                 //Employment details tab

                 employeeID.Text = dread["EmployeeNumber"].ToString();
                 startDate.Text = ( dread["IOfficialStartDate"] != DBNull.Value ? Convert.ToDateTime(dread["IOfficialStartDate"]).ToString("dd/MM/yyyy") : "");
                 endDate.Text =  (dread["IOfficialEndDate"] != DBNull.Value ? Convert.ToDateTime(dread["IOfficialEndDate"]).ToString("dd/MM/yyyy") : "");
                 jobTitle.Text = dread["JobTitle"].ToString();
                 empLevel.Text = ( dread["EmploymentLevelID"] != DBNull.Value ? dread["EmploymentLevelID"].ToString() : "-1");
                 opLevel.Text = (dread["Default_OperationalLevelID"] != DBNull.Value ? dread["Default_OperationalLevelID"].ToString(): "-1");
                 officeLocation.Text = dread["ICubeOfficeLocation"].ToString();
                 empVisaDate.Text = ( dread["EmploymentVisaDate"] != DBNull.Value ? Convert.ToDateTime(dread["EmploymentVisaDate"]).ToString("dd/MM/yyyy") : "");
                 phoneLine.Text = dread["IDirectLineNumber"].ToString();
                 phoneExt.Text = dread["IExtensionNumber"].ToString();
                 empStatus.Text = (dread["EmploymentStatus"] != DBNull.Value ? dread["EmploymentStatus"].ToString() : "-1");
                 entity.Text = ( dread["Employee_EntityID"] != DBNull.Value ? dread["Employee_EntityID"].ToString() : "-1" );
                 reportsTo.Text = (dread["ReportsTo_EmployeeID"] != DBNull.Value ? dread["ReportsTo_EmployeeID"].ToString() : "-1");
                 dottedLineTo.Text = (dread["DottedLineTo_EmployeeID"] != DBNull.Value? dread["DottedLineTo_EmployeeID"].ToString() : "-1");
                 department.Text = (dread["DepartmentID"] != DBNull.Value? dread["DepartmentID"].ToString(): "-1");
                 organization.Text = (dread["OrganizationID"] != DBNull.Value ? dread["OrganizationID"].ToString() : "-1");
                 contractExpiryDate.Text = (dread["EmploymentContractExpiryDate"] != DBNull.Value ? Convert.ToDateTime(dread["EmploymentContractExpiryDate"]).ToString("dd/MM/yyyy") : "");
                 resVisaStartDate.Text =(dread["ResidenceVisaOfficialStartDate"] != DBNull.Value ? Convert.ToDateTime(dread["ResidenceVisaOfficialStartDate"]).ToString("dd/MM/yyyy") : "");
                 domainUserName.Text = dread["IDomainUsername"].ToString();
                 employmentType.Text = (dread["EmploymentType"] != DBNull.Value ? dread["EmploymentType"].ToString(): "-1");

                 //banking details tab

                 bankName.Text = dread["BankName"].ToString();
                 bankSwift.Text = dread["BankSWIFT"].ToString();
                 bankRoutingNumber.Text = dread["BankRoutingNumber"].ToString();
                 bankAccountNumber.Text = dread["BankAccountNumber"].ToString();
                 bankIBAN.Text = dread["BankIBAN"].ToString();

                 //IDs tab
                 laborID.Text = dread["LaborID"].ToString();
                 immigUID.Text = dread["ImmigrationUID"].ToString();
                 emiratesID.Text = dread["EmiratesIDNumber"].ToString();
                 drivingLicense.Text = dread["DrivingLicenseNumber"].ToString();
                 drivingExpiryDate.Text = (dread["DrivingLicenseExpiryDate"] != DBNull.Value ? Convert.ToDateTime(dread["DrivingLicenseExpiryDate"]).ToString("dd/MM/yyyy") : "");
                 passportNo.Text = dread["PassportNumber"].ToString();
                 passportIssueDate.Text = (dread["PassportIssueDate"] != DBNull.Value ? Convert.ToDateTime(dread["PassportIssueDate"]).ToString("dd/MM/yyyy") : "");
                 passportIssuingAuthority.Text = dread["PassportIssuingAuthority"].ToString();
                 passportExpiryDate.Text = (dread["PassportExpiryDate"] != DBNull.Value ? Convert.ToDateTime(dread["PassportExpiryDate"]).ToString("dd/MM/yyyy") : "");
                 residenceVisaFileNo.Text = dread["ResidenceVisaFileNumber"].ToString();
                 residenceVisaExpiry.Text = (dread["ResidenceExpiryDate"] != DBNull.Value ? Convert.ToDateTime(dread["ResidenceExpiryDate"]).ToString("dd/MM/yyyy") : "");
                 laborWorkPermitNo.Text = dread["LaborWorkPermitNumber"].ToString();
                 laborCardExpiry.Text = (dread["LaborCardExpiryDate"] != DBNull.Value ? Convert.ToDateTime(dread["LaborCardExpiryDate"]).ToString("dd/MM/yyyy") : "");

                 //notes tab
                 notes.Text = dread["Notes"].ToString();

                 using (SqlConnection con1 = new SqlConnection(System.Configuration.ConfigurationManager.ConnectionStrings["ApplicationServices"].ConnectionString))
                 {
                     con1.Open();
                     using (SqlCommand com1 = new SqlCommand("SELECT EffectiveDate, DaysOff, YearlyTickets, AverageTicketValue, PhoneAllowance FROM EmployeeBenefits WHERE EmployeeID = " + DropDownList3.SelectedValue, con1))
                     {
                         SqlDataAdapter dad = new SqlDataAdapter(com1);
                         DataTable benefitDT = new DataTable("EmployeeBenefits");
                         dad.Fill(benefitDT);
                         GridView1.DataSource = benefitDT;
                         GridView1.DataBind();
                     }
                     con1.Close();

                 }

             }
             dread.Close();
             con.Close();
         }
     }
 }