I'm unable to find music stored in the Google Play Music app

This issue will be fixed by updating Google Play music.
After deleting the data and cache of Google Play music, please update the application.

1. To Clear data and cache:
   1. Press the HOME button on the remote.
   2. Select Settings.
   3. Select Apps in the TV category.
   4. Select Google Play music in the System app category.
   5. Select Clear data and choose OK.
   6. Select Clear All Data.
   7. Select Clear cash and choose OK.
      
      
2. To update the app:
   1. Press the HOME button on the remote.
   2. Select Google Play store.
   3. Select Auto-update apps in the Settings category.
   4. Select Auto-update apps at any time.
      If you don’t want to Auto-update, after the update has been completed, change the setting to Do not auto-update apps.

NOTE:
In old versions (5.9.18 or older) of Google play music, content stored on external storage devices displayed first. (If you remove the external storage, Google play music content will be displayed.)