ORA-20000

Error:

C:\>sqlplus /nolog

SQL*Plus: Release 11.2.0.1.0 Production on Thu Oct 17 17:55:05 2013

Copyright (c) 1982, 2010, Oracle.  All rights reserved.

SQL> conn SCOTT/TIGER@SIEB306
ERROR:
ORA-28000: the account is locked

SQL> exit

Cause: The user has entered wrong password consequently for maximum number of times specified by the user's profile parameter FAILED_LOGIN_ATTEMPTS, or the DBA has locked the account

Action: Wait for PASSWORD_LOCK_TIME or Unlock the account.

SQL> 
SQL> select dba_users.username, dba_users.account_status
  2  from   dba_users
  3  where  dba_users.username = 'SCOTT';
 
USERNAME                       ACCOUNT_STATUS
------------------------------ --------------------------------
SCOTT                          EXPIRED & LOCKED
 
SQL> ALTER USER SCOTT ACCOUNT UNLOCK;
 
User altered
 
SQL> 
SQL> select dba_users.username, dba_users.account_status
  2  from   dba_users
  3  where  dba_users.username = 'SCOTT';
 
USERNAME                       ACCOUNT_STATUS
------------------------------ --------------------------------
SCOTT                          EXPIRED
 
SQL>

See also: ORA-28001: the password has expired

And prevent account locks to not occur again:

sql> ALTER PROFILE "DEFAULT" LIMIT PASSWORD_LIFE_TIME UNLIMITED;
sql> ALTER PROFILE "DEFAULT" LIMIT FAILED_LOGIN_ATTEMPTS UNLIMITED;