Rene,
That's not an issue we've ever heard of. Can you let us know the following:
1) What version of Joomla are you using?
2) What version of JFBConnect were you upgrading from (if you remember)?
3) If you re-install JFBConnect, do the menus appear? You should be able to re-install it without uninstalling first. You won't loose any settings or previous data.
4) Does the login button, or any other JFBConnect features, seem to wok on the front-end still?
Thanks, and sorry for the troubles,
Alex