popup button click event is not firing while popup inside the div

<div id="popupdiv" runat="server" style="display: none" >

                          <table>
                              <tr>
                                  <td><span class="txt">Username: </span><span class="redstar">*</span></td>
                                  <td>
                                      <asp:TextBox ID="txtUserName" runat="server" placeholder=" UserName"
                                          Width="186px"></asp:TextBox>
                                      <asp:RequiredFieldValidator ID="rfvUserName" runat="server"
                                          ErrorMessage="Please enter  User Name" ControlToValidate="txtUserName" ValidationGroup="Login"
                                          Display="Dynamic" ForeColor="Red"></asp:RequiredFieldValidator>
                                  </td>
                              </tr>
                              <tr>
                                  <td><span class="txt">Password: <span class="redstar">*</span> &nbsp;&nbsp;</span></td>
                                  <td>
                                      <asp:TextBox ID="txtPwd" runat="server" TextMode="Password" placeholder=" Password" Width="186px"></asp:TextBox>
                                      <asp:RequiredFieldValidator ID="RequiredFieldValidator1" runat="server"
                                          ControlToValidate="txtPwd" Display="Dynamic" ValidationGroup="Login"
                                          ErrorMessage="Please enter Password" ForeColor="Red" SetFocusOnError="True"></asp:RequiredFieldValidator>
                                  </td>
                              </tr>
                              <tr>
                                  <td></td>
                                  <td>
                                      <asp:Button ID="btnSubmit" ValidationGroup="Login" runat="server" Text="Login" OnClick="btnSubmit_Click"/>
                                      <asp:Button ID="btnclose"  runat="server" Text="Cancel" CausesValidation="false" style="height: 26px" /></td>
                              </tr>

                              <tr>
                                  <td>&nbsp;</td>
                                  <td>&nbsp;</td>
                              </tr>

                          </table>
                    </div>