1.在cmd中输入sqlplus/nolog:
2.connect username/password as sysdba;会显示连接（管理员不用密码登录）
3.改密码：alter user you_username identified by you_password;
eg:alter user system identified by 123456
（oracle）invalid username/password:logon denied
原文：http://blog.csdn.net/u011707402/article/details/46670029