I know this may sound simple, but did you accidentally plug the keyboard into the mouse port and vise-versa? If so, it would explain why both a ps/2 and a usb keyboard wouldnt work.
Also, i dont think you are able to get to this option in your current state, but in the bios, USB keyboards might be disabled.
I dont know how you would fix this, its kind of like the forgotten password kind of deal, "forgot your password?, please enter username and password to continue".
Anyway, thats getting off topic.
The only other reason I would think that a PS/2 keyboard would stop working is a IRQ setting conflict. Did you change anything in the bios recently or add/change any kind of hardware/software/drivers just before the keyboard stopped working?