Not all reviews can be re-enabled, but for those that can, I suggest the reviewer have the option to do that on their History tab.

A review could only be re-enabled if (1) it is a bulk review order (i.e. the author/publisher ordered more than one review) AND (2) the total number of reviewers who have selected the book is less than number ordered.

In other words, when a reviewer cancels their review (or it gets canceled and marked as "not complete" for some other reason such as the reviewer not updating their status on time) the book is then re-listed for other reviewers to select for review, meaning other reviewers have probably picked it up and it is no longer available for any reviewers to select for review, including the one who canceled it. In that case, which is the more common case, we cannot allow the reviewer to re-enable.

However, in the rare case where a previously canceled review can be re-enabled, I suggest a button appear on the reviewers History tab that says "re-enable" to allow the reviewer to re-enable that review, basically as if they were re-selecting the book for review.