Does your Kobo turn off and on normally? Or is it stuck loading the book and totally not responding? If so, reset your Kobo. Look on the bottom edge of the Kobo - see the little pinhole? Stick a paper clip into there until you feel it push against a little switch. Remove the paper clip.

Is the Kobo working, but simply can't open a book? If it's only affecting a particular book, try deleting it and downloading it again.