Try this one its may help to you..
login page button event use below code
if (con.State == ConnectionState.Closed)
{
con.Open();
}
SqlDataReader dr = null;
SqlCommand sqlcmd = new SqlCommand("your select query", con);
dr = sqlcmd.ExecuteReader();
if (dr.Read())
{
Session["UserName"] = txtusername.Text;
dr.Dispose();
}