My kid's PC just did something similar. I see the PS2 keyboard connection has failed on that one so I put in some USB keyboard for a quick fix till we either upgrade the machine or I find time or incentive to pull the motherboard out to repair it.
I upgraded my video card to a Raedon 1900xt and for some odd reason my mouse and keyboard have decided not to work, they come on briefly during boot up, but then shut right back off. Now I have removed the card to see what happens and still no mouse no keyboard. Please help I don't know what I could have done I checked all the other plugs and everything is plugged in.