If you can still access the USB file system after a reboot, place a file called ENABLE_DIAGS (*all caps, no file extension!!!*) in the root directory of the device, and reboot again. Provided that you have the rescue pack installed, this should get you into diags mode, from where you can try to debrick it.
Otherwise... SOL, I guess. There are no known methods to debrick a Paperwhite, besides possibly soldering and accessing it via serial port.
PS: Moved this out of the Kubrick thread and into a new one, as Kubrick can't do anything about Paperwhites.