GRUB runs off the BIOS devices. You'll need to go in to the BIOS (probably with a PS/2 keyboard) and enable "Legacy support for USB devices" or whatever it is called in your BIOS.
That is the solution. I was able to get into the BIOS without using my PS/2 keyboard -- my cordless worked just fine. And the option is called Legacy Support for USB Devices
. Made it really easy to fix the problem.