Following instructions will help you unlock the oracle db system user and lock it back when needed.
Those are the instructions:
1) login to a machine as user oracle
2) connect to sqlplus: sqlplus '/ as sysdba'
3) unlock user: alter user system account unlock;
4) change password: alter user system identified by manager;
5) verify connection: conn system/manager
6) restore password: ALTER USER system IDENTIFIED BY VALUES 'F2B917B76C7DFF0A';
7) lock user: alter user system account lock;
8) verify lock: conn system/manager
you should see the following message: ERROR: ORA-28000: the account is locked