errors while retrieving records from database using 3-tier architecture?

public DataSet FindRecord(int RollNumber)
      {

              using (SqlConnection con = new SqlConnection(cs))
              {
                  DataSet ds = new DataSet();
                  SqlCommand cmd = new SqlCommand("spfindtblstudent", con);
                  cmd.Parameters.AddWithValue("@RollNumber", RollNumber);
                  SqlDataAdapter da = new SqlDataAdapter(cmd);
                  cmd.CommandType = CommandType.StoredProcedure;
                  con.Open();
                  da.Fill(ds);
                  con.Close();
                  return ds;

              }
      }

public DataSet Find(int RollNumber)
       {
           DataSet ds = null;
           Dal objdal = new Dal();
           ds = objdal.FindRecord(RollNumber);

           return ds;
       }

protected void btnfind_Click(object sender, EventArgs e)
        {
            DataSet ds = new DataSet();
            int RollNumber = Convert.ToInt32(txtid.Text);
            Bll objbll = new Bll();
            objbll.Find(RollNumber);

            if (ds != null && ds.Tables.Count > 0 && ds.Tables[0].Rows.Count > 0)
            {
                txtname.Text = ds.Tables[0].Rows[0]["Name"].ToString();
                txtaddress.Text = ds.Tables[0].Rows[0]["Address"].ToString();
                txtphone.Text = ds.Tables[0].Rows[0]["Phone"].ToString();
            }
            else
            {
                Label1.Text = "No Record(s) avalilable";
            }