Long shot: Did your Wi-Fi network properties change on the Asus laptop or Fire tablet? For example, if you are running a dual band Wi-Fi router (very common), did the Fire tablet or Asus laptop "spontaneously" switch from 2.4GHz band to 5GHz band, or vice-versa? Sometimes that can happen if your router or network clients are not set up to force a single band, and device discovery for some devices may be sensitive to it. I've never run JRiver for Android, but I know for a fact that my old Chromecast Audio renderers can only be discovered on the 2.4GHz band (hardware limitation). If the MC Server switches to 5GHz, I've seen those devices "disappear" from the MC server zones.
Did the Asus go to sleep? Is it running both Media Center 27 and Media Center Service? IIRC there were some significant improvements to MC Service between Versions 27 and 30.