Searching data from database using checkbox list for a ecommerce website

<div>>
        <asp:CheckBoxList ID="chkUserData" runat="server" AutoPostBack="true" 
            onselectedindexchanged="chkUserData_SelectedIndexChanged" 
            RepeatDirection="Horizontal">
        </asp:CheckBoxList>
        <asp:GridView ID="gvUserDetails" runat="server" EmptyDataText="No Record Found">
            <RowStyle BackColor="#EFF3FB" />
            <FooterStyle BackColor="#507CD1" Font-Bold="True" ForeColor="White" />
            <PagerStyle BackColor="#2461BF" ForeColor="White" HorizontalAlign="Center" />
            <HeaderStyle BackColor="#507CD1" Font-Bold="True" ForeColor="White" />
            <AlternatingRowStyle BackColor="White" />
        </asp:GridView>
    </div>

 protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {
SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings["dbconnection"].ConnectionString);
            con.Open();
            SqlCommand cmd = new SqlCommand("select UserId,UserName,Email from UserMaster", con);
            cmd.ExecuteNonQuery();
            SqlDataAdapter da = new SqlDataAdapter(cmd);
            DataSet ds = new DataSet();
            da.Fill(ds);
            for (int i = 0; i < ds.Tables[0].Rows.Count; i++)
            {
                chkUserData.Items.Insert(i, new ListItem(ds.Tables[0].Rows[i][1].ToString(), ds.Tables[0].Rows[i][0].ToString()));
            }
        }
    }

  protected void chkUserData_SelectedIndexChanged(object sender, EventArgs e)
    {
 if (!String.IsNullOrEmpty(chkUserData.SelectedValue))
        {
        SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings["dbconnection"].ConnectionString);
        con.Open();
        SqlCommand cmd = new SqlCommand("select UserId,UserName,Email from UserMaster where UserId='" + chkUserData.SelectedValue + "'", con);
        cmd.ExecuteNonQuery();
        SqlDataAdapter da = new SqlDataAdapter(cmd);
        DataSet ds = new DataSet();
        da.Fill(ds);
        gvUserDetails.DataSource = ds;
        gvUserDetails.DataBind();
    }
        else
        {
            SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings["dbconnection"].ConnectionString);
            con.Open();
            SqlCommand cmd = new SqlCommand("select UserId,UserName,Email from UserMaster", con);
            cmd.ExecuteNonQuery();
            SqlDataAdapter da = new SqlDataAdapter(cmd);
            DataSet ds = new DataSet();
            da.Fill(ds);
            gvUserDetails.DataSource = ds;
            gvUserDetails.DataBind();
        }
}