[Solved] Bootstrap Modal Popup Auto Closes on PostBack in ASP.Net

You will have to re-open it after PostBack.

Refer

[Solution] Bootstrap Modal Popup Auto Closes on PostBack in ASP.Net

thanks sir ,

i have created code for reopen bootstrap model popup click on inside save button.

in source code.

  <form id="form1" runat="server">
    <asp:ScriptManager ID="ScriptManager1" runat="server">
    </asp:ScriptManager>
    <div class="container">
        <div class="row">
            <h1>
                Hello, world! Hello, world!</h1>
            <!-- jQuery (necessary for Bootstrap's JavaScript plugins) -->
            <script src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.2/jquery.min.js"></script>
            <!-- Include all compiled plugins (below), or include individual files as needed -->
            <script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.2/js/bootstrap.min.js"></script>
            <button type="button" class="btn btn-primary btn-lg" data-toggle="modal" data-target="#myModal">
                Launch demo modal
            </button>
            <asp:UpdatePanel ID="UpdatePanel1" runat="server">
                <ContentTemplate>
                    <!-- Modal -->
                    <asp:Label ID="Label1" runat="server" Text=""></asp:Label><br />
                    <div class="modal fade" id="myModal" tabindex="-1" role="dialog" aria-labelledby="myModalLabel"
                        aria-hidden="true">
                        <div class="modal-dialog">
                            <div class="modal-content">
                                <div class="modal-header">
                                    <button type="button" class="close" data-dismiss="modal" aria-label="Close">
                                        <span aria-hidden="true">&times;</span></button>
                                    <h4 class="modal-title" id="myModalLabel">
                                        Modal title</h4>
                                </div>
                                <div class="modal-body">
                                    <asp:TextBox ID="TextBox1" runat="server" placeholder="First Name" class="form-control"></asp:TextBox><br />
                                    <asp:TextBox ID="TextBox2" runat="server" placeholder="Middle Name" class="form-control"></asp:TextBox><br />
                                    <asp:TextBox ID="TextBox3" runat="server" placeholder="Last Name" class="form-control"></asp:TextBox><br />
                                </div>
                                <div class="modal-footer">
                                    <button type="button" class="btn btn-default" data-dismiss="modal">
                                        Close</button>
                                    <%--<button type="button"  class="btn btn-primary">
                                        Save changes</button>--%>
                                   <asp:Button ID="Button1" runat="server" Text="Save" OnClick="Button1_Click" />
                                      
                         </div>
                            </div>
                        </div>
                    </div>
                </ContentTemplate>
            </asp:UpdatePanel>
            
                                     
        </div>
    </div>
       <script type="text/javascript">
           function openModal() {
               $('#myModal').modal('show');
           }
</script>
    </form>

c# code

 protected void Button1_Click(object sender, EventArgs e)
    {
        ScriptManager.RegisterStartupScript(this, this.GetType(), "Pop", "openModal();", true);
        Label1.Text = TextBox1.Text + " " + TextBox2.Text + " " + TextBox3.Text;

    }