Styles go missing constantly

I discovered after the last update (Nov 23/15), the custom styles I created in Word are gone and my document had reformatted to the default styles.  I had previously saved, closed and reopened the document in question several times without issue. I checked per the suggestion and the auto update was unchecked.  All styles are visible.

Is this still Word 2013? I don't have a "drop down menu" in the Stylesgroup. This is the Styles group I have:

If I open the Styles pane (using the dialog launcher in the bottom right corner) and click on the New Style button, I get this:

I can't find a way to generate the dialog you show. To make your new style available in future documents, you would select the radio button for "New documents based on this template."

Yes, there's definitely something wrong there. Unless you have intentionally removed the other styles from the Quick Styles gallery, they should be displayed. As a check, open the Styles pane, right-click on, say, Heading 1, and see whether it says Remove from Style Gallery or Add to Style Gallery. If it says Add, then of course you can add it and the other missing styles. But if (as is more likely), it says Remove, then there's definitely something wrong, and I can only suggest that you repair the installation.

Are these built-in or custom styles? What do you mean by "go missing"? Are they not listed in the Styles pane? Or are they no longer applied to the appropriate paragraphs? Or are they still applied but no longer have the formatting you defined?

If the problem is the last, it could be that "Automatically update document styles" is enabled. If you modified styles but saved the changes only in the document and not in the template, then every time you open the document, the styles would be reset to their format in the template. If that's the case, then on the Developer tab, in the Templates group, click Document Template. In the Templates and Add-ins dialog, make sure "Automatically update document styles" is not checked. (If you don't have the Developer tab displayed, you can display it by checking it in the Customize the Ribbon dialog.)

If the problem is just that the styles disappear from the Styles pane, then perhaps you need to change the Style Pane Options to display "All styles."

Thank you for the quick response, sorry I haven't replied sooner.

What I meant by "go missing" is that all the styles in the 'Styles group' in the Home tab disappear, apart from the list paragraph style which seems very strange.

I did implement your suggestion to turn off automatic updating but it was already turned off, so unfortunately the problem is still here.

The image attached is just to show there is only one style available to select. But there are heading, title and normal styles in the document that were all selected from the style box. After saving, closing and re-opening the document only one remains available.

Hope you can provide a solution, thanks for all help regardless of the outcome.