After 1 week of investigations we find out that a page from our site was modified to contain spam words. We changed all passwords, remove spam from the site and now the traffic from Google is back again. Stay alert, WordPress hackers are everybody.
That's why I started to use all non-standard software. When you use super popular blog, forum, and CMS programs there are so many hacks that your data is never safe. I use paid blog software that not many people use, it just cuts the number of hackers down 90%. It's not 100% hack proof, but there are many less hackers coming at you.