I just got Windows 8.1 set up via Boot Camp a couple days ago, and everything was working wonderfully.
Today, I upgraded my Mac to OSX v10.9, as suggested by Apple. Nobody told me it would break my Boot Camp system!
First, my Windows partition didn't even show up in the option-menu at boot, so I tried following the instructions here, which fixed the option-menu, but the system still won't load!
When I tried to boot into Windows, it says it can't find the operating system! So somehow it seems that the 10.9 update is incompatible with previously-existing Boot Camp partitions.
What can I do? I need my Windows partition for work!