I was transferring files to Kindle's SD card via USB when it locked up. Neither power cycle nor hard reboot fixed it. Instead it would start the power-on cycle, put up the happy Amazon Kindle logo but then hang forever. Was about to call Amazon when I tried powering it down, popping out the SD card, and then booting it. Voila! Kindle came up fine. Turned it back off, re-inserted the SD card, and everything is working fine again.