This usually means you accidentally created two different Brilliant.org accounts. Perhaps one with your email, and another via the Log in with Google or Facebook workflows. You can learn how to diagnose this issue, close one of your accounts, change your email address on your account, and other account changes here [link].
Occasionally, this can also be caused by an issue with your underlying payment provider (PayPal, credit cards, etc.). If you're certain that you have only one Brilliant account and that you should still have access to Premium, please reach out to us with the email address associated with your Brilliant account at email@example.com. We'll be able to get back to you much faster if you can associate the purchase receipt or transaction ID with your email.