If you are talking about the actual server machine, depending on the type of device and operating system, it’s something like this procedure: https://support.plex.tv/articles/204281528-why-am-i-locked-out-of-server-settings-and-how-do-i-get-in/
If it’s Windows, you can also use this procedure: You need to reclaim your server, but don't dare to touch the Windows Registry
