private void EnterAcNotextBox_TextChanged(object sender, EventArgs e)
        {
            int id;
            if (!int.TryParse(AcNotextBox.Text, out id))
            {
                return;
            }
            string connString = ConfigurationManager.ConnectionStrings["dbx"].ConnectionString;

            string cmdString = "SELECT '#' FROM tblBasicInfo, tblAccData WHERE ID =" + Convert.ToInt32(AcNotextBox.Text);

            //string query = "SELECT * FROM tblAccData WHERE ID =" + Convert.ToInt32(AcNotextBox.Text);

            using (OleDbConnection conn = new OleDbConnection(connString))
            {
                using (OleDbCommand cmd = new OleDbCommand(cmdString, conn))

                {
                    conn.Open();

                    OleDbDataReader reader = cmd.ExecuteReader();

                    if (reader.Read())
                    {
                        StudentNametextBox.Text = (reader["SName"].ToString());
                        FatherNametextBox.Text = (reader["FName"].ToString());
                        ClasscomboBox.Text = (reader["ClassofReading"].ToString());
                        TransportFatextBox.Text = (reader["TranFacility"].ToString());
                        RemarkstextBox.Text = (reader["Remarks"].ToString());
                        CelltextBox.Text = (reader["Cell"].ToString());
                        TrantextBox.Text = (reader["TransportChr"].ToString());

                        DuestextBox.Text = (reader["Dues"].ToString());
                        AdmfeetextBox.Text = (reader["Adm"].ToString());
                        ProFeetextBox.Text = (reader["Pro"].ToString());

                    }
                }
            }

From my opinion your problem should be in construction of sql statement i.e."SELECT '#' FROM tblBasicInfo, tblAccData WHERE ID =" + Convert.ToInt32(AcNotextBox.Text)

Use Inner join clause to join two tables.
You can try it

"SELECT A.ID,A.Name,.....,B.Pro FROM tblBasicInfo A Inner Join tblAccData B On B.ID=A.ID WHERE A.ID =" + Convert.ToInt32(AcNotextBox.Text)

***Use Parameterised queries to prevent sql injection to your database.