try this
if (!IsPostBack)
{
BindGrid();
}
public void BindGrid()
{
Con = new SqlConnection("Your Connection Details");
Con.Open();
dt = new SqlDataAdapter("Select * from table", Con);
DataSet ds = new DataSet();
dt.Fill(ds);
GridView1.DataSource = ds;
GridView1.DataBind();
Con.Close();
}