If your Word TOC broke and you receive the message Error! Bookmark not defined
You may change a heading in your Word document, to find out that your Table of Contents broke and instead of showing the page number for the new heading it gives you an error message, that the bookmark is not defined.
If you right click your TOC you are not given the option of updating page numbers only, Word updates the complete TOC, replacing the existing TOC along with all the special formatting you might have applied before.
Before you create a new Table of Contents and lose all the special formatting of your previous table, try to repair manually the problematic entry in your old TOC.
Just delete the page number of the specific entry in your old TOC, then go to the References menu and click the Cross-reference option. In the dialogue that appears select the “Heading” option at the left and the Insert Reference to “page numbers”, at the right, then locate the heading in your document that you want to include in your TOC.
Word now enters the page number for your new heading and your old TOC is updated keeping all your special formatting! Enjoy!