SQL> ALTER USER HR ACCOUNT UNLOCK;

SQL> alter user hr account lock;

User altered.

SQL> conn hr@pdborcl/hr
ERROR:
ORA-28000: the account is locked


Warning: You are no longer connected to ORACLE.

SQL> conn system@pdborcl
Enter password:
Connected.
SQL> show user
USER is "SYSTEM"
SQL> alter user hr account unlock;

User altered.

SQL> conn hr@pdborcl/hr
Connected.
SQL> show user
USER is "HR"
SQL>