I've actually had the same issue occur now on a separate domain controller. It looks like when the ATP Sensor went to self update, it broke itself during the install process. And again the same result - it reports as not responding in the portal. On the Domain Controller, there is no ATP service listed in services.msc, and the sensor is unable to be uninstalled (because it doesn't exist) and it's unable to be reinstalled because it thinks it already is.