I had the same thing last week. My Apple TV is connected via ethernet, yet did not show a valid IP address. Rebooting, resetting the Apple TV didn't help.
I ended up using a Micro USB cable to connect the Apple TV to my Mac and restoring it in iTunes.
The cable doesn't come with the Apple TV (any doesn't the Apple TV use the regular 30 pin connector???), so you will have to get one before you can do the restore - or take it in to the Apple Store.
After the restore everything was fine...