If you’re on the server itself, click on the Plex icon in the task bar, or go to http://127.0.0.1:32400/web.
If you’re on a different system, point a browser at the IP address of the server. For example, if the address is 10.1.2.3, use http://10.1.2.3:32400/web.
Also: Why am I locked out of Server Settings.
And: You need to reclaim your server, but don’t dare to touch the Windows Registry
