Login to SQL Plus client on the oracle database server machine.
enter user-name: system
enter password: password [Only if, if you have not changed your default password while DB installation]
press enter. after which, you will be seeing the connection status.
Now,
SQL> ALTER USER [USER_NAME] ACCOUNT UNLOCK;
press enter.
you will be seeing message: user altered.
Now try login with the user name on db client[sqldeveloper].