in special circumstances blocks can vanish completely.
Check the following procedure:
- You need the block "Navigation" configured with the following options:
- Original block location: System
- Page contexts: Display throughout the entire site
- Click on "Turn editing on".
- Go to "Navigation / My profile / Messages.
- Click on "Configure Navigation block".
- Go to the setting "Display on page types" and set it to "Any message pages".
- Save this configuration.
- The navigation block is vanished!!!
You can not find it in the "Add a block" list or anywhere else.
The only two way I found to get the block back is:
a) Change the pagetypepattern to "*" in the table "block_instances" inside the database.
b) Remove this block in the blocks configuration and add it again.