Login with the credential of sys or system User or with any other Account who is having oracle DBA rights. Once we logged in with the oracle DBA User, type the below comment to unlock the account.
SQL>ALTER USER USERNAME ACCOUNT UNLOCK;
e.g SQL> alter user HR account unlock;
We will get the message 'account altered' once we executed the above command.
Once you unlocked the user you can login with the user if you have password of the unlocked user name. If suppose you forgot the password or do not the know password,
use the below comment to change the password.
SQL>ALTER USER USERNAME IDENTIFIED BY PASSWORD;
e.g SQL> alter user HR identified by HR;
Now you can login with the username and pasword by using below comments.
e.g SQL> conn HR/HR@ORCL
Hope it is useful.