Button with popup does not execute procedure

I have a button in each row of a table, that at the moment of pressing it must generate a popup with some txtbox in which the data of the row where the button is pressed is loaded, but it does not execute the procedure to load the data in the textbox

table where the button is

                <Columns>

                    <asp:TemplateField HeaderText="Detalles">
                        <ItemTemplate>
                            <div class="col-md-5">
                                <asp:Button ID="btnVerdetalles" runat="server" Text="Ver Detalles" CommandName="BotondeDetallesdeNombres" CommandArgument='<%# Container.DataItemIndex %>' 
                                Onclick="cargardatos" CausesValidation="False"/>



                                <ajaxToolkit:ModalPopupExtender ID="ModalPopupExtender1" runat="server" TargetControlID="btnVerdetalles" Enabled="true" CancelControlID="btnCerrarPopUp"
                                    BackgroundCssClass="modalBackGround" PopupControlID="Panel1">
                                </ajaxToolkit:ModalPopupExtender>
                            </div>
                        </ItemTemplate>
                    </asp:TemplateField>
                </Columns>
            </asp:GridView>

Panel where the textbox is

                             ×                 Details                                                                                                                      

                            <asp:GridView ID="tabladenombresexternos" runat="server">

                            </asp:GridView>
                        </div>

                        <div class="control-group">                                            
                                <label class="control-label" ><strong>Nombre :</strong></label>
                                <div class="controls">
                                    <asp:TextBox ID="txtAliasNuevoUsuario" name="txtAliasNuevoUsuario" type="text" placeholder="" class="form-control input-md" runat="server"></asp:TextBox>
                                    <ajaxToolkit:FilteredTextBoxExtender
                                    ID="txtAliasNuevoUsuario_FilteredTextBoxExtender1"
                                    runat="server"
                                    BehaviorID="txtAliasNuevoUsuario_FilteredTextBoxExtender"
                                    TargetControlID="txtAliasNuevoUsuario"
                                    FilterType="LowercaseLetters, UppercaseLetters, Numbers" />
                                </div>
                        </div>

                        <div class="control-group">                                            
                                <label class="control-label" ><strong>Contraseña :</strong></label>
                                <div class="controls">
                                   <asp:TextBox ID="txtContraseñaNuevoUsuario" name="txtContraseñaNuevoUsuario" type="password" placeholder="" class="form-control input-md" runat="server"></asp:TextBox>
                                   <ajaxToolkit:PasswordStrength ID="PS" runat="server"
                                    TargetControlID="txtContraseñaNuevoUsuario"
                                    DisplayPosition="RightSide"
                                    StrengthIndicatorType="Text"
                                    PreferredPasswordLength="10"
                                    PrefixText="Seguridad:"
                                    TextCssClass="TextIndicator_txtContraseñaNuevoUsuario"
                                    MinimumNumericCharacters="0"
                                    MinimumSymbolCharacters="0"
                                    RequiresUpperAndLowerCaseCharacters="false"
                                    TextStrengthDescriptions="Muy Debil;Debil;Promedio;Fuerte;Excelente"
                                    TextStrengthDescriptionStyles="cssClass1;cssClass2;cssClass3;cssClass4;cssClass5"
                                    CalculationWeightings="50;15;15;20"
                                    Enabled="true"
                                    BarBorderCssClass="border" />
                                 </div>
                          </div>

                    </div>
                </div>
            </div>

        </div>
        <div class="modal-footer">

            <asp:Button ID="btnIngresarUsuarioNuevo" runat="server" OnClick="btnIngresarUsuarioNuevo_Click" Text="Ingresar" />
            <ajaxToolkit:ConfirmButtonExtender ID="ConfirmButtonExtender1" runat="server" ConfirmText="Desea continuar con la operacion?" TargetControlID="btnIngresarUsuarioNuevo" />

         </div>

the event that I want to summon to load the data in the textbox

protected void cargardatos(object sender, EventArgs e)
        {
            tabladenombresexternos.DataSource = objeto.TraerUsuarioCredenciales(Convert.ToInt32(tabladeusuarios.SelectedRow.Cells[1].Text.ToString()));
            tabladenombresexternos.DataBind();

            if (tabladenombresexternos.Rows.Count < 1)
            {
                string script = string.Format("alert('{0}');", HttpUtility.JavaScriptStringEncode("El usuario seleccionado no posee credenciales"));
                if (Page != null && !Page.ClientScript.IsClientScriptBlockRegistered("alert"))
                {
                    Page.ClientScript.RegisterClientScriptBlock(Page.GetType(), "alert", script, true /* addScriptTags */);
                }
            }
            else
            {
                txtAliasNuevoUsuario.Text = tabladeusuarios.SelectedRow.Cells[3].Text.ToString();
                txtContraseñaNuevoUsuario.Text = "1234";
                Session["idusuarioseleccionado"] = tabladeusuarios.SelectedRow.Cells[1].Text.ToString();

            }
        }