I had a similar issue when updating the device FW to 4.8.2, via the GUI although bizarrely, it looked to be just my TP-Link mesh APs that had dropped connection, despite picking up IP addresses. All wired devices ‘seemed’ to work as expected.
Anyway, I used WinSCP to replace the AdGuard Home files (the files that are usually backed up had disappeared!). Then, and this was the important part, remove the config.yaml file from the device (/etc/AdGuardHome), start the service via the GUI and run through the initial OOTB set up. Once it’s working, turn off the service again and copy the original config.yaml file back across to the device. Start the service up once more and it was now working as normal.
All credit to this guy: How to upgrade Adguard on the Flint, auto and manual? - #27 by admon
Paul