Assigned block system to an invalid region I've renamed a region in my .info file by mistake.
After clearing the cache with drush, the block I've attached to that region was disabled.
Drush printed the following message:

The block Nivo Slider was assigned to the invalid region slider and has been disabled.

I corrected the false region, but still the block seems to be "disabled" somehow, see picture from structure -> blocks, below.



Already enabled/disabled Nivo Slider Module, cleared cache with drush.
I can't explain why, but if somebody does the same by mistake and assigns a invalid region to the block. The following did the job for me.
Just delete the entire region and add the block to another region. Clearh the cache and afterwards setup the region you had before again and assign the block to it.