If this happens, remember which email address you are registered on Facebook with. You can reset your password by entering the email address here: https://www.iconfinder.com/user/password-reset . You will get a link on that email.
After you are logged in to your account, you can go and reconnect your Facebook account, so things should be working as before.
In order to do that, just use the "Connect to Facebook" button from your Account Settings: https://www.iconfinder.com/account/settings