1) an Oracle sequence cannot be locked.

2) an oracle sequence will NEVER generate an unbroken sequence.

--
-- we create a table to keep our sequence
--
create table number_sequence (aninteger integer);
insert into number_sequence values (0);
commit;

--
-- create a function to get the next sequence value
--
create or replace function f_get_next_number return integer is
   return_v integer;
begin
   update number_sequence set aninteger = aninteger + 1;
   select aninteger into return_v from number_sequence;
   return (return_v);
end;
/
show errors

--
-- use the function in your code
--
begin
   for r1 in (select * from user_tables where rownum < 10) loop
      dbms_output.put_line(f_get_next_number);
   end loop;
   commit;
end;
/

SQL> create table mhe_seqtab(seq_col number)
  2  /

Table created.

SQL>
SQL> create sequence mhe_seq start with 1 increment by 1
  2  /

Sequence created.

SQL>
SQL> insert into mhe_seqtab
  2  values ( mhe_seq.nextval )
  3  /

1 row created.

SQL> insert into mhe_seqtab
  2  values ( mhe_seq.nextval )
  3  /

1 row created.

SQL> insert into mhe_seqtab
  2  values ( mhe_seq.nextval )
  3  /

1 row created.

SQL> insert into mhe_seqtab
  2  values ( mhe_seq.nextval )
  3  /

1 row created.

SQL> insert into mhe_seqtab
  2  values ( mhe_seq.nextval )
  3  /

1 row created.

SQL> insert into mhe_seqtab
  2  values ( mhe_seq.nextval )
  3  /

1 row created.

SQL> insert into mhe_seqtab
  2  values ( mhe_seq.nextval )
  3  /

1 row created.

SQL> insert into mhe_seqtab
  2  values ( mhe_seq.nextval )
  3  /

1 row created.

SQL> commit
  2  /

Commit complete.

SQL>
SQL> select seq_col
  2  from   mhe_seqtab
  3  order  by 1
  4  /

   SEQ_COL
----------
         1
         2
         3
         4
         5
         6
         7
         8

8 rows selected.

SQL> drop sequence mhe_seq
  2  /

Sequence dropped.

SQL> drop table mhe_seqtab purge
  2  /

Table dropped.

SQL>

2) an oracle sequence will NEVER guarantee an unbroken sequence.

SQL> create sequence s;

Sequence created.

SQL> select s.nextval from dual;
   NEXTVAL
----------
         1

1 row selected.

SQL> /
   NEXTVAL
----------
         2

1 row selected.

SQL> /
   NEXTVAL
----------
         3

1 row selected.

SQL> @sys
Connected.
SYS> shutdown abort
ORACLE instance shut down.
SYS> startup
ORACLE instance started.
Total System Global Area  314572800 bytes
Fixed Size                  1296428 bytes
Variable Size             130025428 bytes
Database Buffers          171966464 bytes
Redo Buffers               11284480 bytes
Database mounted.
Database opened.

SYS> @c
Connected.
SQL> select s.nextval from dual;
   NEXTVAL
----------
        21

1 row selected.

SQL> /
   NEXTVAL
----------
        22

1 row selected.