My first Vox died too. It froze, so I shut it off by holding the power button, but after it wouldn't boot ever. I contacted Kobo and they had me do a few things:
1- Hold power and volume up (or down, don't remember which) which launches a factory reset. The reset occurred, but the Vox still wouldn't boot.
2- Battery reset. You have to open the back with your fingernails and press on a button next to the battery DO NOT REMOVE THE BATTERY, it may void the warranty.
After these two attempts failed, they told me to ship the device back and shipped me another one, which I still have.