Installing Ubuntu 12.04 on one of our servers, it turned out that the PS/2 keyboard didn’t work. Luckily, only a few steps were required to make it work - so if you’ve got the same problem, you might give it a try:

$ sudo vi /etc/default/grub

# Change 
# to
#   GRUB_CMDLINE_LINUX="i8042.nopnp"

$ sudo update-grub

And restart.