Error! Bookmark not defined

It would probably be worthwhile to remove the existing TOC completely, and replace it with an automatic TOC (look at this article). The advantage to this kind is that you just have to update the TOC with one click any time the document has been changed.

However, if the document doesn't use the built-in heading styles, or if there are relatively few changes, you could continue with your semi-manual TOC by replacing the missing bookmarks or by changing the references in the TOC.

Where you see the Error! Bookmark not defined message, that's a field. Right-click it and click Toggle Field Codes. You might see something that looks like this:
    { PAGEREF someBookmark }
or something that looks like this:
    { PAGEREF _Ref364868613 }
In either case, the "word" after PAGEREF will be the name of the bookmark that the field originally pointed to. The first kind points to a bookmark that someone inserted by hand, while the second kind points to a hidden bookmark that was created by the cross-reference dialog. Whichever it is, that bookmark no longer exists in the document.

You can use the Insert > Bookmark dialog to create a new bookmark with the old name if that's what you want; or you can just delete the field (select it and press the Delete key) and then use the References > Cross-Reference dialog to make a new reference to a new section, according to these directions.

Thank you - that worked! :)

Have a great night