Yoast SEO gone

We had installed and configured our Yoast SEO plugin for your website http://www.hiddeniceland.is a couple of months ago but when I was going to update SEO on one of our pages yesterday it was not there anymore. It is not visible from dashboard or anywhere on my admin pages.

When I go to the plugin pages of WordPress there I have the option to install it again, but stops at an error because it’s detecting that it’s already installed. But we can’t see it anymore. Can you please help?

The page I need help with: [log in to see the link]