Recently, I discovered that the number pad wasn’t working on my keyboard. No matter what I tried, I couldn’t get any of the keys to work – the Num Lock key changed nothing. Having freaked out for a while (had I poured a pint of water into the number pad without noticing?), I started searching through the control panel for things that may be relevant.
It turns out that Ubuntu had got itself into Mouse Keys mode, whereby the numpad keys can be used to move the mouse pointer. This handy accessibility option is activated by pressing Shift+NumLock, and it seems I inadvertently pressed it at some point. It’s a simple toggle, so pressing Shift+NumLock got me my number pad back again.