Do you have a USB keyboard? Does it work fine? Does it also work fine once you boot into your operating system? Does it not work at all during the boot process before loading your operating system?
If your answer is yes to all of the above, then you've very likely been an idiot like yours truly and enabled the Fast Boot feature in your Intel motherboard's BIOS without reading the fine print.
Fast Boot disables USB ports during boot up to speed things up. Usually this is not an issue. But if you forget how to disable it you cannot enter the BIOS to turn it off. And if you never knew that you might need to disable it, then shame on you and thanks for the company.
Turning the blasted thing off is as simple as powering down the system and then giving the power button a good long press until you hear a few beeps. Once you start the PC again, you will see a message telling you that Fast Boot has been disabled (or something like that) and giving you the option of going into the BIOS setup and turning it off permanently.
Hope this helps someone out there!