<asp:ScriptManager ID="ScriptManager1" runat="server">
        </asp:ScriptManager>
        <asp:UpdatePanel ID="UpdatePanel1" runat="server">
            <ContentTemplate>
                <asp:Label ID="Label1" runat="server" Text="Label"></asp:Label>
                <asp:LinkButton ID="LinkButton1" runat="server" CommandArgument="1" 
                        onclick="LinkButton1_Click">LinkButton</asp:LinkButton>
                <asp:LinkButton ID="LinkButton2" runat="server"  CommandArgument="2" 
                        onclick="LinkButton1_Click">LinkButton</asp:LinkButton>
                <asp:LinkButton ID="LinkButton3" runat="server"  CommandArgument="3" 
                        onclick="LinkButton1_Click">LinkButton</asp:LinkButton>
            </ContentTemplate>
        </asp:UpdatePanel>

------解决方案--------------------
ModalPopupExtender1.Show();

------解决方案--------------------
你是通过点击GridView中的按钮来弹出ModalPopupExtender吗

------解决方案--------------------
[code=C#][/code]
  protected void GridView1_RowCommand(object sender, GridViewCommandEventArgs e)
   {
       if (e.CommandName == "Reject")
       {
           int rowIndex = ((GridViewRow)(((Button)(e.CommandSource)).Parent.Parent)).RowIndex;
           ModalPopupExtender2.Show();
           ModalPopupExtender1.Hide();
       }
       if (e.CommandName == "Detail")
       {
           ModalPopupExtender1.Show();
           ModalPopupExtender2.Hide();
           BindModalUp(e.CommandArgument.ToString());//自定义方法绑定数据，传参数
          
       }
   }