I've been writing a longish book (over 800 pp) with numerous headings and so far link to previous has been working fine. However after I inserted the TOC (itself very longish - 12 pp) the link to previous button greyed out and is no longer working. I've tried inserting a new page before the TOC but this does not help.

A TOC would normally be in the document's first Section, so there wouldn't be a 'previous' to link to.

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]

The fact that you have multiple headers (footers) is not sufficient for the Link to Previous option to be active. Instead, a new section is required.

Each type of header or footer (first page, odd page, even page) can only be linked to/unlinked from its "neighbor" in the preceding section.

In other words, you have to add a section break, so that the current section 1 becomes section 2.

__________________
Stefan Blom
Microsoft Word MVP

Microsoft 365 apps for business
Windows 11 Professional

I've marked this as solved with the two previous answers.
No header/footer in the first section of a document will ever have an active "link to previous" button.
I know that the Mac version is not identical to the Windows version but believe the following resources should still be helpful:
Generating a Table of Contents - Complex Documents
How to create a table of contents in Microsoft Word by Shauna Kelly
Header/Footer Settings Recap
What is the difference between headings and headers?