Well, that leaves trying everything else again:
- Press the power button normally
- Press and hold the power button for up to a minute (normally it forces shutdown after ~15s or so)
- Press the reset button normally (make sure the paper clip is straight and not bent; the reset button gives a slight click feedback if you concentrate on it so you know you're pressing it correctly)
- Press and hold the reset button (not sure if that would ever do anything, but...)
- Touch the display everywhere (probably won't help but ...)
- Do all of that again (once with, once without USB cable connected)
- Wait for the next clear sky full moon and expose device to moonlight for 4+ hours
If nothing helps, sure, get a warranty replacement.
Oh: The WiFi indicator is turned on in the display, if your router can display WiFi clients you could try pinging the Kobo's IP. If it's online with WiFi and stays that way the battery could drain quickly after all. But if it was still alive this far it should also react to the buttons
The LED next to the power button shows no reaction either? If it blinks but the display stays unchanged, it would be a dead display for some reason