Cannot login to Oracle11g with System username I just installed Oracle 11g into my machine.
I was about to shut down a database instance, and Oracle asked me for username and password, I provided the credentials I used to login into the web system, and it said that the credentials were wrong.

I googled how to change credentials, so I logged in into SQL Plus and did the following:

   ALTER USER sys IDENTIFIED BY "my_new_password";


then I tried to shut down the database instance logging in with the user sys and the new password and oracle still complaint about wrong credentials. Then I logged off, and when I tried to log in with my username and password they didn't work. I tried to login with sys and the new password and it didn't work, and now I completely out of the database management web site, and I can't do anything.

I do not know where to go from here, I'm working in a Windows 7 Machine.
You mentioned you were on a Windows machine and a brand new DB.  A problem I've run into is the ORA_DBA group.  Windows has a habit of not allowing you to log into an Oracle DB with high level credentials if your OS account is not in the ORA_DBA group.  We found this problem when we migrated to Windows 2008.  I have no experience with Windows 7, but feel like its a safe assumption that it has a security setup similar to 2008, and this may be your problem.