Left Frame Not Displaying In HTML Output

Problem

I've published my HTML files but the left frame doesn't work. The right frame displays, but in the left frame I get the message "this page can't be displayed" and I can't see the contents of the book.

Reason

Your Index and Table of Contents may be using different types of HTML control - one using "Plain HTML" and the other "HTML Help".

Solution

Change them so they are both the same type, and then publish again.