Please provide Joomla, CB, and FBC versions. This typically happens due to a conflict. Whether jQuery related (review browser error log while using FBC) or PHP related due to failed login, but session failed to clear. Please enable debug mode and maximum error reporting within Joomla global configuration. Once done re-test and review any on-screen errors presented. Same issue was reported happening with CB Connect. It's possible a change occurred on Facebooks end resulting in breaking the usage.