Display ASP.Net GridView based on selected value from DropDownList using C#

Hi,

I am a beginner. I've just started using ASP.NET last month. I got a requirement where I have to display a GridView containing list of information of selected value from DropDownList.

I have been trying few ways that I found from internet but they are not working.

The GridView won't show.

I hope anyone can help me and thank you in advance. 

Here is my front end code

<asp:UpdatePanel ID="select" runat="server">
    <ContentTemplate>
        <div class="col-lg-3">Question :</div>
            <div class="col-lg-9">
                <asp:DropDownList ID="ddlItems" runat="server" CssClass="form-control" AutoPostBack="true" OnSelectedIndexChanged="ddlItems_SelectedIndexChanged"></asp:DropDownList>
            </div>
        <div>
        <asp:GridView ID="selectionItems" runat="server" Visible="true" AutoGenerateColumns="false" CssClass="table table-striped table-bordered table-hover table-condensed table-responsive">
            <HeaderStyle />
            <Columns>
                <asp:BoundField DataField="Id" HeaderText="ID" />
                <asp:BoundField DataField="items" HeaderText="ITEMS" />
            </Columns>  
        </asp:GridView>
    </ContentTemplate>
</asp:UpdatePanel>

My code in c# page_load

DataTable dt1= new DataTable("SELECT [tb_details_items].[items_id], [tb_items].[question] " +
               "FROM [tb_details_items] INNER JOIN [tbl_items] ON [tbl_items].items_id = [tbl_details_items].[items_id] WHERE [items_id] = " + detailsID + " AND [tbl_details_items].[level] = " + level + " " +
               "AND [tbl_details_items].[cond] = 1 AND [tbl_details_items].[items_id]=" + ddlItems.SelectedValue + "");
selectionItems.DataSource = dt1;
selectionItems.DataBind();
selectionItems.Visible = true;

c#

protected void ddlItems_SelectedIndexChanged(object sender, EventArgs e)
{
    String getquery;
    getquery = ddlItems.Text;
    SqlConnection Conn = new SqlConnection(conStr);
    Conn.Open();
    SqlDataAdapter da = new SqlDataAdapter("SELECT * FROM [tbl_items] where [tbl_items].[items]='" + getquery + "'", Conn);
     
    DataTable dt1 = new DataTable();
    if (dt1.Rows.Count > 0)
    {
        selectionItems.DataSource = dt1;
        selectionItems.DataBind();
    }
 
    Conn.Close(); 
    BindData();
}