I've been experiencing frozen screens quite a bit since the last firmware update. Either it realizes it's stuck and reboots, or I have to slide the power button until I see a light and it'll reboot. Sometimes this will happen two or three times in a row. On the bright side, I can still read the thing.
I had another kobo where the screen froze up completely and it would not do anything. No reset, no reboot, nothing. I took it back to a Chapters and they exchanged it on the spot. This was last September. It was frozen on a book screensaver and then it moved to a blank screen with some fine lines in the corner. They said something like, "Ah, the etch-a-sketch problem." From what I understand, they don't tend to do on-the-spot exchanges anymore.