That is usually caused by either
- an invalid storage location of the Plex data folder is referenced in the Registry
- port 32400 is occupied by another app (FTP servers or Torrent clients are usual offenders). Close/disable these before trying to start Plex server.
Use the registry file in “Method 2.2” in the article You need to reclaim your server, but don't dare to touch the Windows Registry
This support article about server logs explains where to fetch them manually.
Alternatively, follow the procedure for complete uninstallation https://support.plex.tv/articles/201941078-uninstall-plex-media-server/
