I have recently lost access to my Atlas 890 after a prolonged power outage. All lights show GREEN and no Alarms are present on the front of the device. The network link however is not showing any activity/lights.
It's difficult to say what is going on without logging into the ATLAS. Can you get a serial connection to either the from CRAFT port or the ADMIN port on the back of the controller module?
It's possible the unit factory defaulted, but if it did, the Ethernet port should still be active, just at the default IP of 10.0.0.1, so I believe logging in with a serial connection is your only option at this time. You can look at this guide for connecting to the CRAFT/ADMIN port: Connecting a Modem to the Control In, Chain In, or Craft Port of an ADTRAN Product
You've probably lost the battery in the clock/calendar/NVRAM chip. Some of the 890s have a separate replaceable battery and some require unsoldering a chip on the motherboard. You really need to take the cover off and look to be sure. See this thread for details: https://supportforums.adtran.com/message/25060#25060