The first thing to check is your network and it’s configuration. If the server is unable to communicate with plex.tv then things will not work as expected.
Also, check the Package Center and make sure the server is still running. The server may have crashed during maintenance as I have reported here: Plex Crashing During Maintenance
After verifying that the server is indeed running and your network is working correctly, close your browser and then reopen it and type in the IP address for your server and add the port number for Plex.
For example: 192.168.0.2:32400
Then go to Plex Settings > General and click on the claim button if it is displayed.
Make sure you select the correct server, if you have more than one.
You may also go to Plex Settings > Authorised Devices > Server and remove any errant server entries there. Keep only the most recent entries.
