You would not have started the oracle service.
Go to control panel -> Administrative Tools -> Services and start the service and set the startup type property to Automatic if you want to automatically startup the db when you start up your pc.
Has something changed in your network configuration? I think if you install/create a db when your network config is tcp/ip and then it changes to named-pipes, (or vis versa) you will get this error. Verify the protocal is set properly in your tnsname.ora file.