[oracle@rac2 ~]$ sqlplus sh/sh;
SQL> create table lock_table (a number);

Table created.

SQL> insert into lock_table values(100);

1 row created.

SQL> commit;

Commit complete.

SQL> 
SQL> update lock_table set a=500;

1 row updated.

SQL>

---- Do not commit -----

SQL> select sys_context('USERENV','SID') from dual;

SYS_CONTEXT('USERENV','SID')
----------------------------------------------------
46 <----

SQL>

[oracle@rac2 ~]$ sqlplus sh/sh;

SQL> declare
b number;
begin
select a into b from lock_table for update nowait;
end;
/  2    3    4    5    6
declare
*
ERROR at line 1:
ORA-00054: resource busy and acquire with NOWAIT specified or timeout expired
ORA-06512: at line 4

SQL>

The below query helps to identify the session that has locked the resource that your session is trying to lock.

[oracle@rac2 ~]$ sqlplus / as sysdba

SELECT OBJECT_ID, SESSION_ID, inst_id FROM GV$LOCKED_OBJECT
WHERE OBJECT_ID=(select object_id
FROM dba_objects
where object_name='LOCK_TABLE' and object_type='TABLE' AND OWNER='SH');

select s.sid, s.serial#,s.inst_id, s.program,p.spid from gv$session s , gv$process p
where  p.addr =s.paddr
and   s.sid in (46);

-- OR --

set linesize 140
set pages 100
col username       format a20
col "SID,SESSION#" format a20
col sess_id        format a10
col object format a30
col mode_held      format a10
select     oracle_username || ' (' || s.osuser || ')' username
  ,  s.sid || ',' || s.serial# "SID,SESSION#"
  ,  owner || '.' || object_name object
  ,  object_type
  ,  decode( l.block
     ,       0, 'Not Blocking'
     ,       1, 'Blocking'
     ,       2, 'Global') status
  ,  decode(v.locked_mode
    ,       0, 'None'
    ,       1, 'Null'
    ,       2, 'Row-S (SS)'
    ,       3, 'Row-X (SX)'
    ,       4, 'Share'
    ,       5, 'S/Row-X (SSX)'
    ,       6, 'Exclusive', TO_CHAR(lmode)) mode_held
 from       v$locked_object v
 ,  dba_objects d
 ,  v$lock l
 ,  v$session s
 where      v.object_id = d.object_id
 and        v.object_id = l.id1
 and        v.session_id = s.sid
 order by oracle_username,session_id;
 
To identify the locked rows, use the below query:

set lines 200
col object_name for a30
select do.object_name
, row_wait_obj#
, row_wait_file#
, row_wait_block#
, row_wait_row#
, dbms_rowid.rowid_create (1, ROW_WAIT_OBJ#, ROW_WAIT_FILE#,
                ROW_WAIT_BLOCK#, ROW_WAIT_ROW#) ROW_ID
from    v$session s
,   dba_objects do
where   s.ROW_WAIT_OBJ# = do.OBJECT_ID
and do.object_name=upper('&OBJECT_NAME_WITHOUT_SCHEMA_NAME')
;

-- OR --

SELECT
    O.OBJECT_NAME,
    S.SID,
    S.SERIAL#,
    P.SPID,
    S.PROGRAM,
    SQ.SQL_FULLTEXT,
    S.LOGON_TIME
FROM
    V$LOCKED_OBJECT L,
    DBA_OBJECTS O,
    V$SESSION S,
    V$PROCESS P,
    V$SQL SQ
WHERE
    L.OBJECT_ID = O.OBJECT_ID
    AND L.SESSION_ID = S.SID
    AND S.PADDR = P.ADDR
    AND S.SQL_ADDRESS = SQ.ADDRESS;

SQL> SELECT OBJECT_ID, SESSION_ID, inst_id FROM GV$LOCKED_OBJECT
WHERE OBJECT_ID=(select object_id
FROM dba_objects
where object_name='LOCK_TABLE' and object_type='TABLE' AND OWNER='SH');  2    3    4

 OBJECT_ID SESSION_ID    INST_ID
---------- ---------- ----------
     82844         46          1  <--- Please note session ID 46

SQL> select s.sid, s.serial#,s.inst_id, s.program,p.spid from gv$session s , gv$process p
where  p.addr =s.paddr
and   s.sid in (46);  2    3

       SID    SERIAL#    INST_ID PROGRAM                                          SPID    
---------- ---------- ---------- ------------------------------------------------ --------
        46         11          1 sqlplus@rac2.rajasekhar.com (TNS V1-V3)          5768

SQL>

You can try executing the query/job after some time (to allow the other session to complete). 

--- OR ----

kill the session that has locked the resource

Syntax: alter system kill session 'SID,SERIAL#,@INST_ID'; (For RAC)
alter system kill session 'SID,SERIAL#';(For Single instance)

SQL> alter system kill session '46,11';

System altered.

SQL>


SQL> declare
b number;
begin
select a into b from lock_table for update nowait;
end;
/  2    3    4    5    6

PL/SQL procedure successfully completed. <-- executed without error after killing problematic session 

SQL>