Penny2019,
While you haven't mentioned what steps you've completed, if you haven't already try signing out of Apple Music and then signing back in.
Keep in mind, this will remove any songs made available for offline play, and you’ll need to download them again then when you log back in.
To sign out, navigate to Settings > [Your Name] > Media & Purchases > Sign Out.
Once you’ve signed back in, turn on Sync Library:
Turn on Sync Library with Apple Music
You can also turn on the automatic download option to quickly download content when added to your library.
- "Always download music: Go to Settings > Music, then turn on Automatic Downloads.
Songs you add are automatically downloaded to iPhone."
You can find that information here: Add music to iPhone and listen offline - Apple Support
If the issue continues, or again if you've already attempted that step, reach out to Apple Support directly to further assist.
Contact - Official Apple Support
Contact Apple for support and service - Apple Support
Cheers!