1. Connect as sysdba from the command line:
sqlplus / as sysdba
2. List all users:
select username from dba_users;
3. Check whether the user account is locked:
select * from dba_users where username = 'USERNAME';
4. If locked, use this command to unlock the account:
alter USER USERNAME account unlock;
5. To change the user’s password type the following command:
password USERNAME;