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I use Plex as well and it is still showing up in My Media.
I would put the TBox in to deep sleep mode, restart the Plex server, then restart the TBox and see if that fixes the problem.
Can you see the Plex server using any other device?
I could suggest a complete system reboot in the following order:
Shut everything down (sequence doesn't matter here - try disconnecting the power from the TBox to ensure that it has fully switched off).
Start up your router/modem and leave for 5 minutes to fully reboot.
Start your Plex Server and leave another 5 minutes.
Start you TBox and see what happens (cna take a few minutes to find everything again)