I haven't changed it in /admin/structure/blocks and it is still there.
All that I can think that I installed a few modules to try them out and then disabled them and maybe that caused it.
In any case, my users can't log in and I am in big trouble.
How to go about debugging the problem? Thanks.
Update: I am not logged in (I see "Log in or register to post comments"). Even when I load /user/logout the login block is not there (not in the HTML)
The "recent log messages" shows nothing out of the way.