The most common reason for your Premium subscription suddenly disappearing is due to having a second Spotify account created using Facebook.
The best thing to try is logging out of Spotify, then back in using your email address and password. Avoid using the ‘Log in with Facebook’ button, as this will log you back into your Facebook-created account.
If this works and you’re now seeing your Premium account, it’s worth closing your free account to avoid any confusion in the future.
To confirm, you should see this (the Spotify Premium box under "Current Plan") in your Account overview if you're on Premium: