This morning when I started my Windows Phone 8 emulator to test an application, the emulator refused to work, giving me this error "The Windows Phone Emulator wasn’t able to connect to the Windows Phone operating system". Luckly it's very easy to fix. It's caused by the virtual network interface that has been disabled for some reason (well... in my case it must depend on the other day when I tryed to trick WP7 firmware upgrade and used the disconnect trick, but this is another story).
I searched on Google for a solution and I landed on this page http://pauliom.com/2012/12/20/the-windows-phone-emulator-wasnt-able-to-connect-to-the-windows-phone-operating-system/
All you have to do is going to "Network and Internet --> Network Connections", right click on "vEthernet (Internal Ethernet Port) Windows Phone Emulator" and Enable it.