much of my music is greyed out and won’t play across devices

There are two alternate services, iTunes Match and Apple Music. iTunes Match allows for songs and playlists to be matched or uploaded to an iCloud Music Library, where they become available to download or stream to any compatible device or computer where you are signed in with your Apple ID. Apple Music is an extension of this service where you can also add in content from the Apple Music catalog to your iCloud Music Library. When your subscription ends you lose access to any of the content added from the Apple Music catalog, and any of your own content that is stored only in the cloud rather than locally on your computer or device.

If your subscription is currently inactive you won't be able to turn on Sync Library (also known as iCloud Music Library in iTunes) and if it is on already then any cloud content shown is likely to be inactive. Either renew the subscription quickly (I believe you have at best 30 days) or turn those options off to stop displaying the content that is no longer available to you.

See Recover your iTunes library from your iPod or iOS device - Apple Community for options to pull media from any devices that you own into your local library. iOS device backups do not include media. When you say "An old iPod stores most of my music and I’ve backed it up to my pc and then to the Cloud" which Cloud service are you talking about? Your iCloud Music Library is only available if you have an active subscription to iTunes Match or Apple Music. Music saved in iCloud Drive, OneDrive, Dropbox, etc. isn't automatically available to iTunes, and iTunes doesn't work well with third party cloud content if you add it manually from cloud shared folders.

tt2

8Lht3W-KmJ wrote:

I have an iTunes subscription (or did until yesterday when ironically my payment failed to go through - and I doubt I’ll be renewing it) but despite paying out for years I can’t access most of my music because it’s greyed out.

Subscription to what? Do you mean Apple Music or the iCloud service?

• • if you mean Apple Music, then as soon as the subscription ends, so does you access to the music
  • if you mean the iCloud service and it's music that you own, you will need to re-start the subscription and either continue the subscription or download your music from the cloud to your computer, before ending the subscription

As an aside:

I have LOTS of music, both mine and downloaded...

What do you mean by "your" music, as opposed to downloaded music? Is the music you describe as "mine" music that you have created/played and then recorded?

Close iTunes. Delete the iTunes Library Genius.itdb file from your iTunes folder. Restart iTunes. This file should be recreated from data in the .itl and you should find the library not recognized error goes away.

tt2