You can't cancel an ISBN, nor do you want to. If your original publisher is out of business and no other firm bought up its rights, rights revert to you. You now can go to other publishers, point out how well the book was selling (we can hope), and if this inspires them to take it on. You should have an agent. If a new publisher does take on your book, their edition will get its own (new) ISBN, since the third set of digits in an ISBN (the five digit set) is unique to each publisher, just as the first two sets (three digits and one digit) tell the nation and language the book is published in.
