troubleshootting question about keyboard

I have a MS ergonomic keyboard (PS/2) I've been using for quite a few
years now. Until a few days ago, it worked as expected. However, now
the OS (Win 8.1 Pro 64-bit) does not recognize it. I have not changed
my hardware nor have I added/removed any software. My USB mouse works.

When I enter BIOS during the boot up procedures, the keyboard works. I
can move around BIOS. I searched my BIOS settings looking for legacy
devices and did not find anything.

Once my OS starts, the keyboard is not seen. For example, I've tried
typing, function keys, using Ctrl+, and so on. Nothing happens.

What I've done so far:
1. use Device Manager to remove and then re-install the driver
2. unplugged the keyboard, turned off the pc, booted up, shut down,
plug the keyboard back in and boot up
3. scavenge a PS/2 keyboard from another computer to see if that
would work. It did not.

Is the problem hardware related? If so, the mobo or the keyboard?

I've thought about buying a PS/2 to USB adapter to use for the
keyboard, but I don't know if that would work. I'd like to keep using
the existing keyboard (the ergonomic design does make a difference to
me) if the problem is with the PS/2 connector on the mobo. But if the
pc recognizes the keyboard (when I'm in BIOS), it doesn't seem like
it's the mobo PS/2 connector.

FWIW, I've connected a wireless keyboard from my other pc till I get
this problem fixed.

Thanks,

John