PS2 Troubleshooting

Environment: Compaq Deskpro EP Series running 2000 prof sp4

Hi,

Have had a user come to me this morning regarding that her mouse and keyboard aern't functioning - so the machine won't boot (keyboard error) etc.
This is strange, cos no one changed the mouse and keyboard ps2 cables round, but I decided to swap them round anyways, so the mouse was plugged into the keyboard port and vise versa - oddly enough, this sorted the problem, but why has this suddenly occurred overnight?
I had this problem with another machine a few months ago and did the same, which also sorted the problem out - but it seems strange! :-\

Anyone else had this problem?
Would appeciate some feedback on this - yes, I've done a google search, but with not much joy.

Thanks,

Rich.