Plex is not working

I’ve been using Plex forever, nothing has changed on my end.

Now, whenever I try to open it on our Firestick it shows “No Server Found”

So I go to my comp and open Plex. It keeps telling me to download Plex Media Server (Which I already have) and making me download and install it. Which doesn’t fix the issue. Continues to search for media server.

All my movies/shows/libraries are gone.

Did your internet provider recently switched on support of the IPv6 protocol?
You could try to disable it again in the network adapter properties of your server machine.
Then stop/close/quit Plex server, and reboot the server machine. Wait a few minutes, then reboot the firestick as well.