Login page in asp.net redirecting to default page

vs 2010, sql server 2008

I want login page code.my code is not working.

I have user table with username and password.

 code of me is as below

  obj = new common();
            con = new SqlConnection();
            con = obj.getconnection();
            SqlCommand cmd = new SqlCommand();
            SqlDataReader rd;
            cmd.Connection = con;       
            con.Open();

            cmd.CommandText = "select username,password from user where username =@username and password=@password";
            cmd.Parameters.AddWithValue("@username", txtusername.Text.Trim());
            cmd.Parameters.AddWithValue("@password", txtpassword.Text.Trim());
            cmd.ExecuteNonQuery();
            rd = cmd.ExecuteReader();
            if(rd.Read())
            {
                Session["username"] = rd["username"].ToString();    
                if (Session["username"].ToString() == "nithin")
                Response.Redirect("Default.aspx");
            }
         
             else
             {
                 ClientScript.RegisterStartupScript(Page.GetType(), "validation", "<script language='javascript'>alert('Invalid Username and Password')</script>");
             }
        }

when username and password is correct , it should redirect to default page.