Page becomes blank after click on the RowCommand Button inside the Gridview

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ASP.NET

C#3.5

Gridview

Dear friends I have a issue in my codes. I am doing gridview insert ,update,delete events program.
I took one footer and displayed two textboxs and one Add button in the
I have bound gridview.and displayed on the webform1.aspx properly.and on the RowCommand Event.
When I click on the add button for insert the values into my table from the Gridview’s rows. So my current page becomes blank. And everything becomes hide from the current page . why ???

My code are

protected void GridView1_RowCommand(object sender, GridViewCommandEventArgs e)
        {

            if (!IsPostBack)
            if (e.CommandName == "ADD")
            {


                Button Button1 = (Button)GridView1.FooterRow.FindControl("Button1");
                TextBox txtfooterid = (TextBox)GridView1.FooterRow.FindControl("txtfooterid");
                TextBox txtfootername = (TextBox)GridView1.FooterRow.FindControl("txtfootername");
                SqlConnection con = new SqlConnection("Data Source=.;Initial Catalog=master;Integrated Security=True");
                SqlCommand cmd = new SqlCommand("insert into tab1(id,name) values('"+txtfooterid.Text+"','"+txtfootername.Text+"')", con);
                SqlDataAdapter da = new SqlDataAdapter();
                da.SelectCommand = cmd;
                DataTable dt = new DataTable();
                da.Fill(dt);
                GridView1.DataSource = dt;
                GridView1.DataBind();
            }
        }