I'll first quote my upstairs neighbour:
"Have to try boot with another KB."
If you only have keybord access via bios, then:
Reset bios to defaults.
Change relaed setting you find in bios on keyboard and usb devices.
(An odd thing if you want to try:
I dunno if you have desktop, laptop, but if you have a laptop then try remoing the battery, and only run on cable.
I know it sounds strange but I've had to deal with 3 similar cases, where the mouse and keyboard don't work until I got out the batery. And I let it run only on cable in which cases the mouse and keyboard would work fine... try this if you have a laptop, even if it sounds weird...)