The Plex ecosystem has been split into three apps. You’ll need to download PlexAmp from the Apple AppStore, login using your Plex credentials and then you’ll be able to regain access to your music. Just to be clear, your music files themselves are safe on whatever PC or server hardware you chose to keep it on. Your music is safe, it’s just that accessing it from Plex the main Plex app is no longer available. Think of it as Apple deciding to break apart iTunes into separate apps (Apple Music app, Podcasts app, AppleTV app). That’s exactly what Plex did.
Plex App - For LiveTV and DVR, Movies, and TV show Library’s
PlexAmp - For everything audio such as Music Libraries and Audiobook Libraries.
Plex Photos - Photo’s