Find password expiry date for users, and password lifetime for their profiles.
Execute as sys:
select u.username , u.account_status , u.lock_date , u.expiry_date , u.profile , p1.limit as password_life_time from dba_users u , ( select * from dba_profiles where resource_name like 'PASSWORD_LIFE_TIME' ) p1 where u.profile = p1.profile order by 1

Set password lifetime of a profile (in this example profile 'default') to unlimited: alter profile default limit password_life_time unlimited; commit;

And finally, to unlock a user run the following sql as SYS or SYSTEM: alter user user_name identified by some_password account unlock; commit;