If you find that in KDE your keyboard has suddenly become unresponsive but the mouse and other aspects of your desktop work fine, did you just hold your shift key down for a few seconds? If so you have accidentally activated a feature called slow keys. Most people don’t know it’s there, and I’m not sure why it can be activated by gesture by in the default setup.
To fix the problem, see this post. Yes it’s an Ubuntu forum post, but this is a KDE issue, not a distro-specific issue and this was the most succinct explanation for the fix that I found on doing a search.
kde keyboard problem - Ubuntu Forums
On Kubuntu it popped up a dialog asking me it I wanted slow keys activated. openSUSE KDE gave me no such notification. Fortunately I remembered the existence of the slow keys feature.
A search of this forum showed that a few people have hit this problem, but there was no FAQ for it, hence this post.