JFB login stopped working

Not sure. Try the following:
1) Enable the "Show Facebook API Errors on Front-End" setting in the Facebook API tab. Let me know if there are any errors.
2) Turn off all caching (Global config and System - Cache), just in case.
3) Turn off all SEF URLs and test.

Obviously, none of that should be required, so we can help you re-enable everything, but something above will hopefully make it start working, and we can go from there.

Thanks,
Alex