I had the same problem, mine was caused by having a loose USB pin. When you plug the USB cord into your Kobo does it wiggle around (Don't force it, you'll know if it's loose)?
Try pluging it into your computer, does that unfreeze it? If not you'll have to have to do a hard reset (Everything on your Kobo will be deleted) to do that you slide and hold the power switch while pushing the home button, hold both for about ten seconds, the blue light on the top next to the power slider should start flashing like a strobe. That should unfreeze it.
If the USB port is wiggly you'll have to open a ticket with Kobo support to get it fixed under warrant. Mines in the mail going to have the warranty work done (hopefully they will replace it)
Last edited by The Terminator; 10-13-2011 at 01:28 PM.