I've never had to, but you can do a hard reset by poking the small hole on the back of the device with a paper clip.
http://kobo.intelliresponse.com/inde...set+my+eReader
There is also a method of holding down the home key while sliding the power switch. That is referenced in another forum post from a Kobo employee:
https://www.mobileread.com/forums/sho...2&postcount=14