I have had the same problem. I am using windows 7 and this should also work for windows 8. The services responsible for the TNS listener and database were stopped when I looked in the Task Manager.
1: Hit Ctrl-Alt-Del and select "Start Task Manager".
From there go to the "Services" tab.
There are 5 services that need to be running for the database to be accessed correctly.
In the list below "databasename" is the database name you assigned during creation without the quotes.
I am also using database 11g, if you are using another type those fields would be different below. the * after home is the home install, if you have more than 1 database there would be home1, and home2
These are the 5 services
If any of these services are stopped right click on them and start them. After they have all started go back into SQL developer and re-try the connection and it should work.