4.5.2 style.css is empty.

An odd one: the call is there in the code but the file is empty.
I thought it was the theme but it happens after a few changes are made to every theme I’ve tried, including default WP ones. Deactivating plugins makes no difference.
If I enter the path to the stylesheet, Chrome finds it fine but won’t load the site. Firefox finds an empty file.
It happened first using a 4.4 version so I updated core and same problem.