OnSong Premium is an account-based service. When you first create an OnSong Premium account, you are automatically signed into your account. You can access your account by going into the Utilities Menu in OnSong by tapping on the gear icon in the Menubar and then choosing the Account option.
If you are not signed in, or do not remember your password for editing your account, you can reset your password on the OnSong website.
If you are trying to sign into OnSong, you can also reset your password by tapping on the Reset Password link if your log in fails.
If you are trying to access add-ons that you had access to originally, you can learn how to restore access to those features.