Ofttimes you see someone trying to do some Oracle database administration, but can't seem to login or start the database.
Just go to https://localhost:1158/em
Disregard the warning and choose proceed/always accept.
Login with username 'SYS' (without the quotes)
Enter the password you provided during installation, then make sure you create your own administrator account.
If the oracle database is down and will not startup, go to Start, Run, enter 'services.msc', look for oracledbconsoleorcl and start service. Also start the oraclelistner service too. This will solve the startup problem. But if the service will not start, you probably were connected to a network during installation, this made oracle associate your computer with the IP address you were using during installation, hence you will need to be connected to a network with the same IP address for the service to start. You can install microsoft loopback adapter and configure it with the IP address Oracle associated your computer with (this IP address will sometimes show when your restart your computer, in a command prompt environment for emca or you can go to your oracle_home directory, db2/db1 subdirectory, oc4j subdirectory, and j2ee subdirectory, you will see a folder with its name starting with the ip address).
And if this is much trouble for you, you can just reinstall the oracle database software (it will indicate that the folder is not empty, don't worry just go ahead, because deleting the folder is not quite straight forward, you will have to delete them subdirectory by subdirectory, there is one folder that will refuse to delete). But make sure you are not connected to any network be it wired or wireless.