Page becomes blacnk after click on the RowCommand Button ins

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 <EditItemTemplate>.

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();

}

}