Oracle sort order changed
NLS_LANGUAGE determines the sort order. If you've went from, for example,  FRENCH to the defaut, AMERICAN, you would get the difference in sort order you describe. To set the default NLS_LANGUAGE execute 

alter system set nls_language ='FRENCH' scope=spfile;


and bounce the instance. You can always override this in the session using

alter session set nls_language = 'FRENCH';


To bounce the instance is to restart it. You need to do that when changing some system parameters like NLS_LANGUAGE. If you only change the parameter in the current session you don't need to restart the instance.

To check the parameter values you can, like Phil said earlier, query NLS_DATABASE_PARAMETERS, NLS_INSTANCE_PARAMETERS, and NLS_SESSION_PARAMETERS. To check the setting in the current session you can also run show parameter nls_language.