Have you tried a manual factory rest? The instructions are
here. You want the last method.
It could be a broken screen, though you should be able to see lines on the screen showing this.
Otherwise, reimaging the internal SD card might solve it. I have sent you a PM with the link to an image to try.
As to warranty, Kobo stopped selling these in October 2016, so it probably is past warranty. You can try for the stationery warranty, but that will depend on exactly when you bought it and what they consider a normal life for a device like.