Write the person and explain the situation and ask if they will please remooch the book. Otherwise, they could send you a point through charity or the smooch button.
I have sent several messages but the other person isn't responding at all. It seems like this could not be the only time this has occurred. Kind of demoralizing that sometimes you'll just lose a book.
Try emailing them from your email program (their email address will be in the original request email if you still have that). Some email providers don't always accept Bookmooch emails.
