What you could try is to perform a clean reinstall. Make sure to follow each step of the process. Then log out and back in.
If this doesn't fix it and the song appears grayed out, it is likely due to unavailability in your country. Availability can change over time and between countries, depending on the permissions from rights holders. We do however add new music on Spotify every day. The missing conent might soon reappear!
Hope this helps 🙂
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