Question : Problem: Can a failed/failing motherboard ruin keyboards?

Two weeks ago I plugged a hard disk, via USB cable/adapter, into my test machine. When I plugged the USB cable into my test machine (with the second hard disk attached), my test machine abruptly powered down. Upon rebooting, my keyboard and KVM switch no longer worked. So, I replaced both the keyboard and KVM.

All has been well. Until about two hours ago.

I connected another hard disk to my test machine tonight, transferred data from that hard disk to my test machine's hard disk with no trouble, and powered down the test machine. Then I powered down the USB adapter that connected the second hard disk to my test machine.

A minute later I noticed my test machine was powered on. But there was no video signal (nor would the keyboard nor KVM connected to the test machine work). So, I plugged the keyboard, mouse and monitor directly into my test machine. I receive no video whatsoever. I even tried disconnecting all peripherals and using the integrated video adapter just to try and get a video signal from the BIOS. Nothing.

Can a failed/failing motherboard ruin keyboards? Is that what's to blame here?

FWIW, I connected the mouse, keyboard and monitor directly into another system and the mouse and monitor work; the keyboard no longer works.

Answer : Problem: Can a failed/failing motherboard ruin keyboards?

let's see if we can limit it a bit more :
you are running now mobo + cpu + 1 ram stick, video card, PS  and no video? then replace : ram, and video; this leaves only mobo and cpu, of which the morte likely is mobo
but the fact that you plug in an usb device when it powers down leads me to believe it has to be sought there : a bad PS for the external case, or bad grounding; check those (or try with another case)