User Login block missing It vanishes suddenly without any notifications today. I am using Drupal 7 with "Mix and Match" theme. I used to working this morning.

Website shows no error at all.

I have tried the following to get it back: 


Find the block table in the database and tried to modify the visibility of the user, login of current theme entry - not working
At admin->structure->blocks, under the current theme. The "User login" is already under sidebar first. I tried remove it and add back, changing the weight - not working
In the other themes the user login block does not show up either, including the default theme Bartik.
Repeat the above steps with a drush cc all command. Not working.


What else could I try to get it back?
Anyway, I found the problem. It was the permission setting. I should set it to "no restriction" rather than restrict to every tick-able role (because guest is not there to tick)