On your Plex Media Server host (the Windows system where you installed it), find the Plex icon in the notification area (aka, the System Tray, you may need to click the ^ to see it). Right click the Plex icon and select “Open Plex.” This will take you to the Plex login screen. When you do, it should log you in and claim your server.
You should then be able to see server again from other apps, including app.plex.tv (once you log them back in).
