Same issue here at end of 2020. Spotify desktop app is unusable. Playlists show but no content within and my Liked songs are empty. On iOS app and Spotify web shows all my liked songs and is syncing (although Spotify Web doesnt see any playlists only my liked songs.) Google searches shows this been going on since 2013 with many people and still no fix.... Thinking of cancelling as unusable on PC.
Have you tried performing a clean reinstall of the app on your PC. This is simple and often overlooked. A clean reinstallation involves deleting the app cache. Check out this guide for the relevant steps.
You can also have a look at the Solutions in this thread and try deleting the local data of your Spotify client. This often causes sync and content issues.
Let us know when you've tried these and if it helped.
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