If you have forgotten your password head to the Fanvue login page and hit the 'Forgotten password?' button. This will take you to a page that will prompt you for your login email. Once completed you will be sent a link to this email to reset your password so you can access your account again.
If you created your account using a Google or Twitter account, sign in with those services. You can add a password to your account by going to Settings > Account > Change My Password