Want to populate datagrid based on dropdownlist changing

I have ddlStudents and dgMultiWhen I make a selection with ddlStudents nothing happens to dgMulti.If I refresh the page manually dgMulti gets populated based on the selectionin ddlStudents.   <%@ Page Language="VB" MasterPageFile="~/sfcas.master" Title="Classes and Students" Debug="true" enableViewStateMac="false" AutoEventWireup="true" %> <script runat="server"> Dim contents As String Protected Friend Sub btnAssignStudent_Click(ByVal sender As Object, ByVal e As System.EventArgs) Dim Conn As New SqlConnection(SqlDataSource1.ConnectionString) Dim intClassId, intStudentId As Integer i ...