If you're on Premium, try signing out and back into the app.
If it says Free, check the receipt we emailed you.
Make sure you're signing in with the username on that receipt. If the username on your receipt is a 10 digit number, sign in with your Facebook login details.
It's also possible that you’ve accidentally created a second account - one account with a Spotify username, the other created and linked to your Facebook profile. Try logging in with each to access your Premium.
For more info and tips about why your Premium subscription might not be working, check out this article.