Hello. No it did not suddenly happen. The site is at https://www.mollywatson.me and was a clean install of WordPress 4. Clean install of themes and other plugins. Right from the start the large icons are not visible in the admin area, or on the live pages. See this screen grab:
https://swearingdad.com/images-temp/no-large-buttons.png
However the small buttons are visible in the admin area and on the live site:
https://swearingdad.com/images-temp/small-buttons-ok.png
I have tried turning off all other plugins and change the theme to standard twenty thirteen one and the issue still persists.
I run an almost identical site https://www.basement-studios.tv where everything is fine, same theme and additional plugins, the only difference is the version of WordPress is 3.9.2 and AddToAny plugin is 1.3.5. I did try to install 1.3.5 on the mollywatson.me site but that did not work.
Do you think it could be a WordPress 4 issue? Or a permissions issue? I have quickly glanced at the plugin folder permissions and nothing seems obviously ‘wrong’.