Changing password doesn't work
I suggest the following:

At first login as SYSDBA and check the ACCOUNT_STATUS of your user (SYSTEM in the sample below).

&gt; sqlplus / as sysdba
SQL&gt; select username, account_status, lock_date from dba_users where username='SYSTEM';


If this statement doesn't return a record, then your account does not exist.    

If the account is locked (maybe because you entered a wrong password several time) then you have to unlock the account:

SQL&gt; alter user SYSTEM account unlock;


To reset the password you can use this statement:

SQL&gt; alter user SYSTEM identified by NEW_SYSTEM_PWD;


If you are working with a development system, then you can ignore case sensitive logons and make sure passwords never expire again:

SQL&gt; alter system set sec_case_sensitive_logon=false scope=both;
SQL&gt; alter profile default limit password_life_time unlimited;


See Oracle documentation for further infos regarding DBA_USERS