We've been having a similar issue with keyboards on 5.1 with no X, and when we upgraded to 5.5 recently we seem to still have it. All HP hardware about 3 years old. You have to unplug the keyboard and plug it into a different port, then return it back to the original to get it back. Sometimes you have to walk around to the other side of the rack to do it.
Very frustrating ... After the holidays I'll get a dmesg with more details
