some of my WP sites have disappeared.

OMG

some of my WP sites have disappeared.

Well, they are there when I look at them, but one of the site owner told me he saw just a blank page, so I checked on my mobile using a different ISP, different browsers with emptying the cache etc, but all fine, apart for one try, where I saw the blank page, but it was there after I did a hard refresh (?)

I then created iFrames for all my WP sites, and some work fine within the exact iFrame, and other, like the one where I saw the blank pages won’t display at all.

I deactivated all plugins, installed the new Twenty Sixteen Theme, but still got the same problem.

I also noticed a difference between a working and non working site. The non working site displays under /wp-admin/update-core.php only:

Last checked on 2015-12-10 at 03:02. Check Again

You have the latest version of WordPress.

Learn more about WordPress 4.4.

Plugins

Your plugins are all up to date.

Themes

Your themes are all up to date.

and one of the working sites displays:

Last checked on 2015-12-10 at 11:04. Check Again
You have the latest version of WordPress. Future security updates will be applied automatically.

If you need to re-install version 4.4–en_GB, you can do so here or download the package and re-install manually:

Re-install Now Download 4.4–en_GB Hide this update

Plugins

Your plugins are all up-to-date.

Themes

Your themes are all up-to-date.

Translations

Your translations are all up to date.

Should I add this form into the broken site to reinstall WP 4.4 and run it? Or how should I go about to get this site back up and running normal again?

What is going on? Some help and good tips would be really appreciated…

Thank you!
Marc