Only thing that worked for me was repeatedly plugging it in and then unplugging it. I would leave it plugged in until the service screen would appear, then I would unplug it. Repeated 5-6x and finally it just booted up normally, though it seems with all caches wiped (e.g. it had all the apps, but no content/images loaded). Also have seen some people say after 5-6x reboots it may bring up a diagnostic screen where you can restore the device to factory. I think Apple doesn't post this as an official solution because its so "inelegant." But it makes sense to have multiple restore options with the USB port removed!