I would try shutting down your device by holding the power button. If it is still stuck maybe try logging out of your account on the device and logging back in. If neither of these work I would try contacting the Spotify service department, as there might be a problem with your account.
Posting here what I heard from Spotify which did in fact solve the issue: Looking at your account from our end, we can see that something occurred with the offline files on your devices. It seems the cache has become corrupted and was disconnected from the app.
Don't worry, if you follow this link you can reset your offline devices. Then you'll need to perform a quick reinstall of Spotify with these steps. This will clear any unnecessary connections between your phone and your Spotify account.
If your tracks or playlists aren't fully syncing after you switch on Available Offline, please check: The device is not in sleep mode. There is enough free space to sync the songs (we recommend 1GB or more). The device has an active internet connection.