How to resolve MS Word "Error! No table of contents entries found" when you build a table of contents?

I have been using an MS Word file for collecting and categorize information. I use headers to denote categories. Over years it has grown to 6+ Mb, 700+ pages and its 2 tables of contents (one level deep, another 4 levels deep) taking over 100 pages together. All of a sudden I started getting this error. 1) Do you... show more I have been using an MS Word file for collecting and categorize information. I use headers to denote categories. Over years it has grown to 6+ Mb, 700+ pages and its 2 tables of contents (one level deep, another 4 levels deep) taking over 100 pages together. All of a sudden I started getting this error.
1) Do you know how to resolve this problem within MS Word?
2) Do you know a software application better suitable for collecting and categorizing textual information that also has a utility for automatic conversion of an MS Word document to its own format?
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