Game glitched :(
I was doing well on hard mode, but the game glitched on me and ruined all that progress.
Apparently after assigning everyone to guard duty to repel a huge zombie horde the game refused to acknowledge anyone was idle. Even after removing everyone from guard duty or doing it selectively, the game told me I couldn't take any actions since no survivors were idle.
Shame...the game is fun and addictive otherwise. But this glitch is quite gamebreaking...