[HarryT]

I'm afraid that's a classic picture of a fractured screen substrate. It's absolutely certain that this is what's happened, although obviously I don't know how it happened. Unfortunately a fractured substrate is not covered by warranty - it's physical damage.

I don't think Kobo devices are any more (or less) likely to suffer from screen fractures than any other brand, but it's one reason that I always keep my readers in a sturdy case.