avoid page reload on click of search button

Question

<!-- ModalPopupExtender GRP-->

    <MM1:ModalPopupExtender ID="mp1" runat="server" PopupControlID="Panel1" TargetControlID="btnPopup1"

        CancelControlID="btnCloseGRP" BackgroundCssClass="modalBackground">

    </MM1:ModalPopupExtender>

    <asp:Panel ID="Panel1" runat="server" CssClass="modalPopup" align="center" Style="display: none">

        <asp:TextBox ID="TextBoxGrpSrch" runat="server" placeholder="Enter a Record"></asp:TextBox>

        <asp:Button ID="btnGrpSrch" UseSubmitBehavior="false" runat="server" Text="search" OnssClick="btnGrpSrch_Click" AccessKey="s" CausesValidation="False" />

        <asp:GridView ID="GridViewGrp" runat="server" AutoGenerateColumns="true" CellPadding="4"

            OnRowDataBound="GridViewGrp_RowDataBound" OnSelectedIndexChanged="GridViewGrp_SelectedIndexChanged"

            ForeColor="#333333" GridLines="None" Width="100%" AllowPaging="True" OnPageIndexChanging="GridViewGrp_PageIndexChanging" PageSize="3"

            EnableSortingAndPagingCallbacks = "true">

            <FooterStyle BackColor="#5D7B9D" Font-Bold="True" ForeColor="White" />

            <Columns>

                <asp:ButtonField CommandName="Select" Visible="false" />

            </Columns>

        </asp:GridView>

        <asp:Button ID="btnCloseGRP" runat="server" Text="Close" />

    </asp:Panel>

    <!-- ModalPopupExtender GRP-->

This is my gridview in popup window.. When I search for a record, popup window disappears but it is working. So I want to make popup still there when I click on search button.

Anyone please suugest me an answer,
Thank U..:)