Need help with overset text in story editor but not in page layout (CS6)

We are building a very long book in InDesign CS6, running under Windows 10. The book is a dictionary of 500+ pages where every line of text is a paragraph; there are only 2 paragraph styles and each page has two columns. We are building this with an InDesign book file which has 3 indd’s in its build.

Today when performing some light edits, one of the indd’s developed a 4-page blank area in the layout, the headers and footers were also blank on those 4 pages, and any attempt to reflow the story results in the program crashing. In fact, anything we try to do now results in the program crashing.

The entire text is present in the Story Editor but that part of the text that used to occupy the 4 blank pages is marked with a red line at the left and the word “overset”.  I attached a screenshot of this.

Rebooting the computer doesn’t help.

I read a similar thread posted a few years ago, but in that one reference was made to an "anchored object". No anchor appears in the Story Editor. of our book.

We wonder why this happened, how to avoid it happening again and most importantly, how to repair the document so we can continue working on it.

If anyone can offer any help or assistance in recovering from this devastating situation, we’d be most grateful.