Yeah your kernel crashed on you.
99.9999% likely an SD card problem.
Get a new SD and also make sure it is Class 10.
(and always make sure you shut it own properly, don't just switch it off)
So I brought up this issue over at the Raspberry Pi support Forums and it sounds like this is probably a kernel bug.
http://www.raspberrypi.org/phpBB3/viewtopic.php?f=28&t=55370...except cgminer does not use the cp210x driver they're blaming in that thread. It uses libusb to do direct USB communications.