MS Word has a set limit on how many errors it can handle with autocorrect and passive spell check functions (passive meaning the squiggly lines under words). Large documents can run up against this limit; my 83k word fantasy novel hit it around 60k words, if memory serves, though it may depend on the computer and available resources. Once you hit that limit, it misbehaves in interesting ways. On mine, it dropped all red squigglies but kept the grammar ones. Occasionally it randomly started doing spelling errors again, most often after reloading the document and skipping to the bottom without scrolling through the rest of the document.
If the above posts about formatting and languages didn't fix the issue, then you might have reached that limit. Here are three options:
1) Split the document up into multiple files
2) Fix the things MS Word is complaining about. Either correct mistakes, add unknown words to the dictionary, or tell it to ignore stuff
3) Use a "better" word processor. Once saved in its .odt format, LibreOffice loads my 83k novel roughly three times faster than Word ever did, the spell check functions worked all the way to the end, it's free, and it doesn't revamp the entire menu system with each new version, so once you learn it you can always use it