Suddenly & for no (to me) apparent reason my keyboard stopped responding just after booting into the Ubuntu desktop. I tried several different keyboards, both USB & PS/2, all of which had the same result. Booting from a Ubuntu live CD allowed me to continue to work with the same keyboards.
I vaguely remembered seeing a message asking if I wanted to enable slow keys after I was tapping away at the keyboard doing something and I responded "No" or so I thought. mouse still working I went into System> Preferences> Keyboard and looked around the keyboard configuration. I found a setting marked slow keys under Accessibility which forces long key presses only to be recognised, unchecked it and regained keyboard function ..
I am just adding this for anyone who might find themselves in similar difficulties and not have a clue what to do or where to go!