Spell Check Keeps Turning Itself off in Word 2016 (Office 365 Home)

The box Doug is referring to is not for an entire document but for selected text. If you have no text selected, it is unlikely to change anything. Start by doing Ctrl+A to select all of your document body's text. Then check the setting under proofing language. The box should be empty. If it is checked or filled in without a checkmark, you need to check it until it is empty.

See Mastering the Spelling Checker which contains information useful to anyone trying to understand how this feature can best be used.

If the steps I outline do not solve your problem, the problem is likely in your Style settings with one or more Styles being set to not check spelling or grammar. Understanding Styles in Microsoft Word

There are two dialogs that deal with spell checking. The one you mentioned in your post is the one about checking spelling in the document.

The one shown above is unlikely to be the source of your problem. Is that the one that keeps checking itself again?

The one that is likely the source of your problem is a setting for selected text and is found under the Review tab.

Here are screenshots of the three states.

I should also add inYes. I keep unchecking it and it turns itself back on. I will uncheck it, type for a while, and note I mistyped something but it is not giving me a red wavy line. When I go back it options, the box is rechecked again.  I should also add that I went into regedit, named the proofing tools something else, restarted Word and it still does it.

My document suddenly stopped doing auto correct as I typed and I spent ages trying to figure it out.  A combination of what was said above helped.  Basically everything looked like it was checked so this is what I did.

Highlighted the whole document, went into the "Review - Language & set proofing language and made sure that English United Kingdom was set to solid blue and ticked.

Went into Account/Options/Proofing and scroll to the bottom and ensure that "Hide spelling errors is NOT checked"

This all worked and suddenly it's all back on showing errors etc!

Under Review>Language>Set Proofing Language, is the a tick in the box for "Do not check spelling or grammar"?

Yes I know. I have done that many many many times to try and solve this problem. Unfortunately, selecting the entire document, then unchecking "Do not check spelling or grammar" doesn't magically turn the spell checker on for the entire document. Hence, my headache.