I used to do that when I was in University. Sometime the computer keyboard does not response and I have unplug it and plug it back in to get it working.
i used to have probs, with older ps/2 boards (pentium and pentium II's) i always envied the macs for their hot pluggable keyboard and mouse... now i've never had a prob with ps/2 ports, though everything's strictly usb now.