Being a relatively new laptop that hadn't occured to me. I'll give it a go.

In reply to a post by GeoffB:

I know about the top left hand corner to enable/disable but it's not that; I suspect another hardware glitch. What do people do who don't have these forums, I wonder!

I suspect you have it covered, but on at least two other makes there is a function key that also does this, which may have been pushed by mistake (Acer = Fn+F7), which could also have been pressed by mistake. It is a toggle so doing it again turns it back on.