I've seen a number of variations of this problem on different forums (Acronis, Macrium, etc.) -- try to boot from a rescue CD and ... no mouse, no keyboard. You're STUCK. I had this problem, obviously, and it was driving me nuts. (I have a Z77 system, as did many - but not all - of the posters with this issue). And it's spooky too... I couldn't restore from a good backup image if my system went belly up.
Browsing around this forum yesterday (gosh I wish it were indexed) - I ran across an answer from Dark Mantis that solved my problem. And it's a simple solution -- I had my keyboard and mouse accidentally plugged into a USB3 port at the back of the computer. As soon as I replugged it into a USB2 only port - voila! Instant keyboard and mouse.
His explanation is that USB3 ports don't get initialized until late in the boot process - apparently after the emergency boot disk does its magic. So - gotta use the old USB 2 ports.
Thanks to Dark Mantis and this forum for solving my issue.