try this:

1. use SVRMGRL
2. set your environment to the instance you want to log into
3. CONNECT/INTERNAL
4. if CONNECT/INTERNAL is not working which mean that your password has changed.
5. If your password is changed and you don't know the password, change the password file at ORACLE_HOME\database\pwd.ora. what you need to do here is delete the old password and recreate the new one using ORADIM

let us know if you still have problem